I would suggest 5 inches of 5-lb memory foam, as a possibility. That's what I sleep on regularly, except that I have 1 inch of more dense standard foam as a base, though I think I could get away without it. I have ordered through Amazon, and through some place up in Michigan, I think. I'm planning to do this again, when I finally find the right used B.
There are plenty of stories online of foam sellers not shipping the density of foam that you have ordered. You have to calculate out how much it should weigh, based on the stated density (per cubic foot IIRC) and dimensions, and weigh it BEFORE you open it... because, if you open it, and if it's not as advertised, you WILL NOT be able to repackage it to a reasonable size. Note, though, that most foam is slightly underweight, due to manufacturing tolerances, and what is considered acceptable by manufacturers and sellers.