Let me break this down for you since suppressors are pretty important for survival (they're basically your best friend when dealing with human camps). You can grab them from popping open car hoods, which is neat because it makes sense these would be lying around in engine compartments, or just buy them from merchants if you've got the credits. They're actually pretty reasonable, running about 30 credits for sidearms, 45 for primary weapons, and 60 for the special ones.

Unfortunately yes repair is impossible but here's the good news: just hit l1 for your survival wheel and r1 to attach or remove them whenever you need. This is really helpful because you can actually save some durability by taking them off during those bigger firefights where stealth isn't really an option anymore.

A lot of players don't realize this, but you can actually extend their life quite a bit by being smart about when you use them (switching between suppressed and unsuppressed depending on the situation saves a ton of wear and tear). Just remember that any vehicle can potentially give you any type of suppressor, so don't waste time hunting specific cars thinking they'll give you specific types. The whole system really makes you think about resource management, which is what survival's all about.