M.J.
This is when it's best to go to a mattress store. If you don't know a reputable one in your area, ask friends for recommendations. If you find a great salesperson, he/she can narrow it down to just what you're looking for. I found it really helpful to talk to a salesperson and get pointed to ones that suited a young child that were in my price range. Ended up getting a mid-range mattress for our daughter ... I'm thinking it was Sealy?