I purchased a 4 inch memory foam mattress from Overstock.com and it was literally been a God send. I can actually sleep without pain most of the time. I just put it on top of my mattress. It makes your fitted sheet kind of wonky but it's well worth it. Also an electric blanket helps, even if you use it just enough to warm the bed nicely.
If I can help with anything at all, all of you please feel free to send a pm.
Hugs to all,
Jan