Hi rachelk,
Post #2 here includes a list of some of the better options in the NYC area. Post #7 here includes a little more detailed description of many of them and categorizes them more into “groups”. Some of these would provide a better place to “test” latex and some would be a great option for a purchase as well.
Post #2 here also includes some links to various posts and threads which will give you some good options, choices, and guidelines for a child’s mattress that may help you as well. Some of these may not be available for local testing but they can help you know what may work best and give you some good reference points to look for and make “value” comparisons.
I like the idea of a 6" latex layer (such as the SleepEz 7000 or the slightly less expensive “special” listed here) because they have the option of adding a topper as the child grows and their needs change. As you mention though … latex is a more costly material although it is also very durable and well worth the price … especially with higher quality latex (especially blended Talalay or 100% natural Dunlop). Post #21 here lists the members here who sell online and there may be some good options here either as a possibility or as a “value” reference point.
You are “wise” to avoid the name brand stores both in terms of quality of materials and value!
Hope this helps and if you have questions along the way feel free to post them.
You have some good choices available in the general area 
Phoenix