Hi raices2,
Welcome to the Mattress Forum! 
You can perform a forum search on Alexandria or Washington DC or Baltimore and see what other businesses have been discussed in those regions which may be helpful to you.
I have 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 vetting and qualification process that is part of becoming an approved member of The Mattress Underground. Such an assumption is unfair to both the consumers seeking assistance, as well as the very businesses and manufacturers who have indeed qualified the be members here 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 quality/value guidelines here.
If you have any questions about specifications or certain products you discover, feel free to post back on the forum and I’ll do my best to be assistive.
The only way to know for certain whether the edges of a specific mattress will be suitable for your specific body type and sleeping style or your preferences will be based on testing the mattress in person (or a mattress that is fairly similar) before a purchase or by actually sleeping on it (if you can’t test it in person before a purchase) and if you are uncertain whether you are one of the few that may “need” firmer edge support then a good return/exchange policy may be a more important part of your personal value equation just in case you are one of the few that needs or prefers a firmer edge. While an edge support system can make for a firmer seating edge, the most important reason is to allow for sleeping to the edge of the product and minimizing roll off. Even the firmest edge support system will break down over time if used as a chair.
There are various types of edge reinforcement systems available, and what type is used depends upon the type of support core used within the mattress. The most common type of support core in an innerspring, and there are various edge reinforcement systems used with innersprings. A foam edge encasement “tub” is commonly used around all types of innerspring units. Some newer pocketed spring units are using firmer springs along the edges of the mattress while still retaining the ability of the mattress to flex and be used with adjustable bed baes. These edge systems are not as firm to sit upon, but work quite well at preventing roll off. There are still other types of form inserts or extruded pieces that are placed along the edges or within the center of spring around the perimeter of spring units, and even extra steel reinforcement units that can be clipped in between the border rod of certain spring units around the perimeter. You’d want to confirm with any retailer you’re considering what specific design they use.
Also, realize that thicker comfort layers within a mattress may mask the benefit of any edge support although it would still have some effect … particularly for sitting which compresses the mattress more.
Phoenix