You could call Steelcase or any authorized distributor and they should take care of you, or you could go with a 3rd party brand that may be significantly cheaper.

I dont know of any reputable 3P or universal casters to recommend, but if you do want official Steelcase Casters with minimal effort, we do have a listing for the hard floor soft-casters on Amazon, shipped directly from Steelcase. I believe we have a listing for the carpet hard-casters too, but cant find it at the time of writing this. If you need those I can try to hunt it down. Hope this helps!

PS. Dont let the 1 star review scare you as that individual has absolutely no idea what he was talking about, and unfortunately Amazon no longer allows sellers to respond/reply to reviews when they are blatantly misleading or incorrect. (Proof)

-Colin at TheBackStore