Sears. We spotted it when it was on a super-sale, then it went off sale (orig. $1300--we were not about to pay that much money for a bed for a 3-yo). We shopped around, and then a month later it went on super-sale again. We got the mattress and box spring for like $500, after rebates. We didn't look for anything special per se, but knew we wanted it to be comfortable and to last since she'll have this for a while.
Re: where did you get your full size mattress for toddler?