Toys R Us isn't actually that bad in that department, at least we've had a few bikes from there that have held up well (for the short time he's used them anyway).
EDIT: Looks like they're short in the bikes for that size though.
That's the thing with kids that age, it's almost ok to get crap because they'll grow out of it anyway. We went through 3 bikes last year.
For something above that check out Sport Check, we got my kid's latest bike there and it seems to be holding up well.
These guys seemed to have good bikes too, and they're an actual bike shop:
http://www.bpcycle.com