FWIW I had a good experience with
Mattress Insider including taking advantage of their 'free exchange' policy (the 'exchange' involved donating the first mattress to a local non-profit which was no problem).
They offer both foam and innerspring mattresses in a variety of 'stock' sizes as well as custom sizes and varying thicknesses / firmness.
In my case I wanted a Very Firm innerspring, based on a phone conversation they suggested one to try with the offer to exchange for a no-extra-cost custom addition of even more coils for more firmness if I wanted that after the trial. I ended up getting the extra coils, I'm very happy with the end result, the whole experience dealing with them was positive.
YMMV.