IKEA does have some great beds (their website shows prices from $80 to $200)!! Once Upon a Child might have some toddler beds, too. Their website is www.ouac.com. Kid to Kid has already been mentioned. I would stay away from beds at Wal-Mart, though...Their furniture does tend to be "cheap", and typically does not last very long (personal experience). You might also want to go ahead and look for a regular twin bed, too, which would last even longer than a toddler bed, not to mention being easier to find sheet sets to fit that are more "big boy" than "baby". As long as you have rails up (and those are usually pretty inexpensive, too), your son should be fine in it.
Oh, don't forget to buy a *waterproof* mattress pad/liner to put underneath his sheets for night time accidents or sickness.
Good Luck with your Hunt!
~J.~