Need Natural Mattress that won't SLOPE!

Hello! We co-sleep and we bought a natural futon mattress mostly made of wool and cotten with some sort of foam in the middle. Unfortunately, it now slopes so there is a mountain in the middle. This makes for nursing in the middle of the night very uncomfortable. We are going to be moving in less than a year and I want to get a really nice cal. King latex mattress then. For now, I want something natural in a Queen size that is affordable. Under $500 would be nice. Big requirement....NO SLOPING!!! Any ideas? I was hoping there were latex futon mattresses but can't find much.....any help? Thanks!!!