Sony Parts and Accessories
used to accept online orders, and I have placed orders for replacement parts and it was really convenient. But the search engine and ordering pages are no longer directly accessible. Looks like they want you to phone instead.
Anyway, here is what you are after:
319884701 EAR PAD (860RX) $15.95
. The search box on that page actually works, unlike the one on the current front page.