Mattress Pad Toppers for Toddler Beds

After going back and forth as to whether or not we should get DS a twin/full size bed or just convert his crib to a toddler bed, we've decided to just do the toddler bed route (especially after finding out the conversion kit to a full size bed has been discontinued). DS sleeps with 3 blankets under him in his crib and will not sleep any other way, so I was thinking of buying a foam mattress topper. Has anybody used these? How well does the sheet fit over it? Where can I get one? Our TRU down here is horrible and that's the only local children/baby store. We'll be in Alpharetta at the beginning of the week and I would like to get one while we're up there. Also, how long have you kept your LO in a toddler bed?