lulu
i got DD's big girl mattress from forty winks and it is a low allergen one.
It wsant cheap (about $600) but it is excellent and will last her well into her teens.

Often the cheap ones are only for toddler weights etc and there fore you will need to replace it as they get much heaper to prevent back problems

best IMO to get a good one from the get go

i just checked the brand is simplee kids and it has a 5 year warranty on it