Hi floatingkeyboards.
Welcome to our Mattress Forum! 
Assuming that you’ve read the mattress shopping tutorial here you seem to be on the right track with looking for a good store to test different mattresses and understand by comparison what type of mattresses or materials you may better like and prefer. Philadelphia has been mentioned quite often on our Mattress Forum and you can perform a forum search here and type Philadelphia om the keywords text box, or any city or area (you would be closer to and consider driving) and see what other businesses have been discussed in that region which may be helpful to you.
We discontinued the provision of listings of potential retailers in various geographic regions (unless they are already approved site members), because of the difficulty in maintaining such lists in a retail landscape that is constantly changing, and most importantly the confusion it was creating with the consumer members who incorrectly assumed that these businesses had indeed gone through the strict qualification process and were approved as members of The Mattress Underground. Such an assumption is unfair to both the consumers seeking assistance, as well as the very businesses and manufacturers who meet the criteria to become Trusted Members of The Mattress Underground.
Whatever business you’re considering, I would always confirm that any retailer or manufacturer that you wish to visit is completely transparent (see this article) and also make sure that any mattress that you are considering meets the Post #13 quality/value guidelines here.
Before visiting any store I’d make sure you’ve read two of the most important posts in the tutorial: post #2 here which has more about the different ways to choose a suitable mattress (either locally or online) that is the best “match” for you in terms of “comfort”, firmness, and PPP that can help you assess and minimize the risks of making a choice that doesn’t turn out as well as you hoped for and post #13 here which has more about the most important parts of the “value” of a mattress purchase which can help you make more meaningful quality/value comparisons between mattresses in terms of suitability (how well he will sleep), durability (how long he will sleep well), and the overall value of a mattress compared to your other finalists based on all the parts of your personal value equation that are most important to you (including the price of course and the options you have available after a purchase if your choice doesn’t turn out as well as you hoped for).
For your stomach and side sleeping position (hopefully, you’ll train yourself to gradually switch to a side sleeping position) you may wish to review the guidelines in this [url=Your sleeping style, preferences, and statistics - Your sleep positions - The Mattress Underground] Sleep Positions Article [url] so that you avoid hyperextension in a swayback position that can cause back issues. Usually, someone who sleeps in the prone (stomach) position needs a surface that is firmer and less forgiving, and I agree with you that incrementally switching to sleeping on your side would be better in the long run but again this introduces a new variable and you may need to take into consideration this future adjustment when you consider any mattress. With the future position change in mind, you may wish to look into something that is good for combination sleeping (primary stomach, secondary side sleeping positions) and which may offer some layer exchange possibilities down the road.
Different sleeping positions are part of what determines how deep a cradle you need and how thick the comfort layer should be. This, in turn, affects the type of support layers that will be appropriate. The layers underneath this comfort layer can help with pressure relief for those who change positions and with thinner upper layers or can be primarily focused on preventing you from preventing your heavier parts from sinking down too far with thicker comfort layers.
Of course, your weight and body profile will also make a difference in how far you tend to sink in and how deep a cradle you need in your mattress so for more on your “statistics”, and how they can affect your choices you can read this article here.
As you are considering a side sleeping position I would also throw into the mix that a suitable pillow is an essential part of good alignment for the head and neck and upper body because the gap between the head and the mattress and the curve of the cervical spine needs to be supported just like all other parts of the spine. Like mattresses … there are certain “needs” that depend on body type and sleeping positions but with pillows, personal preferences play a more important role because the face is much more sensitive to textures, temperature, smells, and other more subjective “feel” based properties of a pillow. There is more about choosing pillows in the pillow thread here and the other topics and sources of information that it links to that may be helpful.
Once you’ve had the chance to narrow things down a little further, feel free to post back with any specific questions and I or any of our Expert Members we’ll be happy to assist you.
Phoenix