Hi Stepherdaisy,
From the sounds of it … the mattress you are talking about is some variation of a memory foam mattress where some people “complain” about the feeling of “sleeping in sand” and some prefer this same feeling. It is a slow response material and there are many companies that make hundreds of different variations. One of the more commonly available versions are the different types of memory foam mattresses made by Temperpedic and one of them may have a similar “feel” to what you are referring to however there are many others available which use similar quality materials and layering which have much better value. So if I had to guess … I would say that the mattress you mean is some type of memory foam mattress using memory foam that is on the more “heat sensitive” side.
All mattresses are a combination of soft (for pressure relief) and firm (for support) and the same mattress can feel very different to different people based on their body type, sleeping styles and individual perceptions as well as how different materials feel for different people. Memory foam in particular can feel both soft and firm depending on the heat and humidity of both the room and the person sleeping on it.
In general … it’s usually more effective to do some initial research into the better local outlets than into the specifics a different mattresses although it may be worthwhile first confirming whether one of the Tempurpedics is similar to what you remember. Some guidelines which may help regardless of the type of mattress you are looking for include …
This article which may help you avoid most of the lowest value choices when you are mattress shopping.
This article which can help you tell the better manufacturers or stores from the ones that are more focused on “selling you” a mattress rather than “fitting you to a mattress”.
There are better and worse quality and value mattresses of every type and knowing the materials in your mattress is the only way to really know its quality and value or make meaningful comparisons with other mattresses. Scanning (not studying) the overviews in the mattresses section of the site will give you enough basic information to ask better questions and help you know how well informed the answers are from each different outlet that you visit.
Post #2 here includes the local and regional factory direct manufacturers that are within 100 miles of you which are likely to have better value.
So to recap … I would start by testing the various Tempurpedic models (which would be widely available locally) to get a sense of whether one of them is similar to what you remember and then once you have confirmed that it is some type of memory foam mattress that has the “feel” you like, then I would make a few phone calls to some of the manufacturers on the list to find out what they carry that may be similar (or any type that your preferred “feel” turns out to be) and visit the ones that are the most helpful and promising based on your phone calls.
I think this would be a good couple of first steps … and feel free to post any questions if you run across any “obstacles” along the way 
Phoenix