Hi jaybackintheday,
Select Foam is one of the members of this site which means that like the other members here I believe they are among the better manufacturers in the country and compete well with the best in the industry in terms of quality, value, and service and I wouldn’t hesitate to make a purchase from any of them that I was confident was a good match for me in terms of PPP and my the parts of my personal value equation that were most important to me.
Of course every manufacturer makes a range of mattresses and some of them may be suitable for you and some may not so I would always make sure that you talk with any online manufacturer to make sure you are choosing a mattress that is the most suitable choice for you out of the mattresses they have available (see mattress firmness/comfort levels in post #2 here).
Phoenix