Hi ab54smith,
Your budget may be a little on the low side for a good specialty foam mattress such as memory foam, gel memory foam, or latex and most of your choices in this budget range may not be as high quality as you may wish. Of course this would depend on your own needs and preferences and the quality and durability of the mattress you are looking for (and this will also be connected to your height and weight, body shape, and sleeping positions).
There are not a lot of options in the El Paso area but some of the better possibilities include …
http://www.popularmattress.com/main.php Local factory direct manufacturer. I know little about them and they have no information about the mattresses they make on their site but they are rated well with the BBB and these are the often the types of outlets that have the best quality and value and are also the most knowledgeable and helpful.
http://furniturerow3.reachlocal.net/locations/locationsByState.jsp?state=Texas Regional factory direct manufacturer. They make a range of mattresses and their traditional polyfoam/innerspring mattresses use better than average quality materials and also have good value. They also carry a mostly latex mattress that has either a latex or memory foam topper and some latex hybrids that also have “better” quality/value than most mainstream manufacturers. I would avoid the mainstream brands they also carry.
http://www.thesleepgallerynm.com/ Retailer. They appear to carry Boyd mattresses which do make some budget memory foam and gel foam mattresses. I would call them to see if they carry any of the types you are looking for in your budget range. I would do a little research here if you go in this direction because Boyd is widely available at a fairly wide range of prices.
Bedroom | El Paso, Las Cruces, Southern New Mexico | Household Furniture Retailer. They appear to carry Corsicana which are a line of promotional (lower budget) mattresses which also includes some memory foam and gel foam models. Again I would call them to see what they have available and if they are in your budget.
I would also make sure that the outlets you talk with or visit are informed about the different materials used in mattresses and are willing and able to tell you the materials in every layer of their mattresses (such as the density of any memory foam or polyfoam). This is the only way to know the quality (and durability) of a mattress. This article may help you to identify some of the qualities of better outlets.
As you mentioned … one other option you have if there are no good quality/value mattresses available of the type you want and that are in your budget in your area would be to consider an online purchase. Some better memory foam online options (among others) are in post #12 here.
Phoenix