C.P.
We just moved our 3 year old to a big boy bed. We heard that because of his weight, it didn't really matter. Plus his crib mattress was firm. So we found a Serta mattress at Big lots for $100. This should last him for a few years, since you're suppose to get a new mattress every 8 to 10 years anyway, then he can help pick out a mattress once he's older. He has been in the bed for almost a week now and we have had no complaints at all. He transitioned very well!
Good luck with yours and hope this helps!