Looking for a good but inexpensive mattress for DD2 who is almost 3. I have a friend with a Sam's Club membership, so I was thinking of looking there, anywhere else I should look? Since she's so tiny, we're just going to start her out with the mattress (and no box spring), but I want something that will last awhile (we'll add a boxspring as she gets bigger and is used to being in a big girl bed).