Is there such a thing as firm mattress pads?

I have a pillow top bed. When I first got it, it was amazing. Now it just plain SUCKS! The bed is way too soft. People have suggested I try putting a plywood between my boxspring and mattress, but since I don't have a boxspring, I don't think that'd do much for me. I have a platform bed for all who may care. I was hoping I might be able to use a mattress pad of some sort, but people tell me these things are to make a bed softer, not firmer. Am I doomed to sleeping on the floor until I can get enough money to buy a new mattress?