Does anyone know where to get WA M4 replacement parts?
 

I'm look specifically for the metal block that holds the buffer lock and selector in place, it's the object on the bottom right-hand corner.

Is there a parts list on the Westerns Arms site or something of the like? Because I was unable to find one.

http://img.redwolfairsoft.com/upload...g/WP110S-L.jpg

If you're referring to the buffer lock many places sell em, if you are refering to the piece that the Pistol grip screw onto only few places have them
Redwolf Airsoft, Boom Arms (make sure tell them NO UPS), Airsoft Global, WGC etc
Buffer lock only available by G&P or Element (aka crappy china copy) the lower piece buffer lock attach to is available as a Kit in Prime Lower Receiver Kit AFAIK.

The only place I know of that sells it is Western Arms. It's about $15 CDN a piece, but I'm not sure if you can order it online through their website. I picked it up in person, you'll have to send them an email.

http://www.gasguns.info/dl/WAparts.pdf

Part# 89.