Unfortunately, most Airsoft brands can get away with not shipping replacement parts because people keep buying their shit and never complain out loud. There are very few companies that have good parts availability (for example, KJW directly and WE through KY Airsoft).
A few years back WGC Shop (
https://www.wgcshop.com/) listed a whole lot of S&T parts. I don't see anything for the Sterling, but you might want to try to ask them.