All right, so here's some information on futon mattresses.
1. If you want to buy one because you'd like to fold it up and save space, and you're going to use it as your primary bed, it's not going to be very comfortable. Folding futon mattresses (which are what you seem to be after) are not designed for full-time, normal use. If you are willing to accept that, bi-fold mattresses are the best when it comes to comfort and compactness. Otherwise, buy a futon mattress that doesn't fold and just push it up against a wall when you need it out of your way.
2. It isn't merely thickness that matters -- density and weight are
extremely important, especially if you're going to be using this as a normal sleeping bed. If you buy something fluffy, it's going to compress anyway over time and become uncomfortable. Cotton mattresses with a foam core (preferably a layered foam core) are probably your best bet, if you want a mattress that's relatively dense, weighty, not too hard, and can be folded. Inner-spring futon mattresses can indeed be the best if you're using them frequently, but they're either impossible to fold or are destined to bend/break if you do buy a folding version. Polyester is okay too, but whatever you do, don't get a solid foam bed (or a bed made mostly of foam). Those are the ones which compress, develop pits, get incredibly uncomfortable, and so on.
3. The general rules of mattress maintenance do still apply. Flip it every now and then, turn it around occasionally, and have you and your significant other swap which sides of the bed you sleep on (this will also be good for your hips, shoulders and spines if you sleep on your side). This will lengthen its life and how comfortable it is for you.
Now, here is the person I bought my frame from:
eBay Store - Room Doctor Furniture Company: Eco-friendly Solid Wood Furnishings, We have Platform Beds, Futon Frames
And here's an inexpensive queen frame:
Queen Size Super EZ Futon Frame: Pine Wood Sofa Bed NEW - eBay (item 310106386515 end time Dec-11-08 18:57:22 PST)
I don't use the term "cheap" here because as mentioned, I have one of these, and their quotes on how much weight it can withstand are true. I've had several friends sitting on it at once, bouncing around and generally being energetic and chaotic. Not a single plank has broken or splintered under normal use or even abuse.