Hi tracyamor,
Unfortunately I don’t have the ability to keep track of all the changing models and prices at each manufacturer in every area of the country but I would suggest spending a day talking to the various manufacturers on the Boston list before you decide on which ones to visit. I would tell them the general type of mattress you are looking for, your budget range, the mattresses you are looking at, and any other information that would be helpful and ask them what they carry that they think would be worth testing and would meet your needs. You will find them to be very helpful and open in most cases. The work you do on the phone before you go out and test mattresses will not only save you a lot of time and aggravation but will also give you a very clear sense of who you would be dealing with and help you connect with the “experts” that can help you the most. I personally wouldn’t visit any manufacturer or retailer without calling them first to make sure they have the knowledge, experience, and transparency to help you make good decisions.
When you are dealing with Costco or other big box stores … you are really just rolling the dice. You will find they have little idea of which of the mattresses they sell may be suitable for your needs and preferences (regardless of quality or value) and if you do decide to go in this direction you would either need to test mattresses locally and then try your best to “match” a mattress at Costco (or another big box store) with a similar design (foam density and thickness in all the layers) or you will need to just go by trial and error and keep buying mattresses and returning them until you find the one that has the PPP (Pressure relief, Posture and alignment, and Personal preferences) that you need. I would personally not take the time to find the specs of a mattress where the specs you need are not easily available from the retailer with a phone call. In some cases you could search this forum or other sources online to try and find the information that they don’t provide or make easily available but this will sometimes take you hours per mattress only to find that either you can’t find it at all or that the value isn’t as good as you thought it was. a forum search on “Novaform” (you can just click this) may bring up some of the information you would need about the materials they use to consider them.
The value of a big box store is that they allow trial and error because of their refund policy but constantly returning mattresses and then trying another one can be frustrating as well. It can also affect the quality of your sleep in the time it takes to discover a mattress isn’t suitable for you and in the time it takes to return a mattress and buy a new one. I personally would rather deal with “mattress people” rather than buying anything from a source that doesn’t really know what they are selling … and in many if not most cases you will also find that the quality and value will be better anyway.
The guidelines I use for memory foam are in post #10 here and there are enough good quality and value mattresses that use 4 lb memory foam or higher that I wouldn’t consider anything that used lower density memory foam. I would also suggest that the base layer should be at least 1.8 lbs (perhaps 1.5 lb in the lower budget ranges).
So you would need to find the specs of all the mattresses you are mentioning from Costco and assess them against the minimum guidelines and against other similar mattresses. You would need to call them to find out any missing information that isn’t on their website (or search online for the specs you need). This is something that the retailer should be doing for you but if you have the time you may be able to find it out for yourself and then decide if a particular design many be suitable for your body type and sleeping position. In general you have a choice between either dealing with experts who already know what you would otherwise need to learn or you will need to become one yourself … and the first option is a lot simpler 
In other words … if you don’t already know exactly which mattress to buy at Costco and why you are choosing it and know the specifics of what is in them so you can make meaningful comparisons to other mattresses that you have tested locally … I would avoid them completely unless you are OK with rolling the dice and experimenting until your number comes up and you find the mattress that works for you. For me it wouldn’t be worth the aggravation of dealing with online merchants that don’t have the knowledge and experience to really help you and the overall value there isn’t as good as most people seem to think anyway when it comes to mattresses. Of course each person may also have a different “value equation” than I do or be more willing to track down specs or buy from a retailer that doesn’t know or disclose what they are selling.
If you do wish to go in that direction … I’d be happy to help you assess any mattress you find there but you would first need to include all the foam density information that they provide you with or that you find with internet searching so I would have easy access to the information I would need to help you make meaningful comparisons without looking them up myself. Unfortunately I don’t usually have the ability or time to be able to track down foam specs for mattresses when the retailer doesn’t provide it to their customers.
In my personal opinion … mattresses are one of the most important purchases you will make and will have a greater effect on your overall well being over the next decade or so than almost anything else you will buy. It’s not a purchase I would leave to the luck of the draw".
I should also mention as well that if your newborn will be sleeping on the mattress that I would tend to avoid memory foam completely. Post #2 here has some of the better forum threads and posts about mattresses and children which I believe may be worth reading if your newborn will be spending time on your mattress.
Phoenix