Carpin has a fairly wide selection of casters (hooded and non-hooded, locking, etc.) as well as glides and caster sockets.
Leggett and Platt office furniture group has a division that has casters as well as bases, arms, and other seating/furniture components.
7th floor at Neocon will have lots of imports. Just make sure they’re BIFMA rated. Mockett is on 7th floor as well, I think.