Hi Dave
we had this problem with the castors on Casper's cage. I found some suitable replacements, being intended for use on hospital beds and similar load bearing items, from a company called RSwww.com. Let me know if you need more details but hopefully putting castors into their search field should find them. If a bird gets it into his head that they are good chew items then that might be a problem but it hasn't occured to Casper YET