The standard part is not a separately available item, and for a while www.airsoftworld.net had various AAP-01 parts for sale from disassembled ones but I never saw the selector part. An after-market one may be an option, for example...
https://www.airsoftworld.net/imperial-custom-precision-action-army-aap-01-qfs-quick-fire-selector.html
... but I've no idea what of the original mechanism it still requires.
Personally I find it intriguing that someone chose to swap in a 3D printed part to lock it to semi, why would you do that if you use it just as a pistol, so maybe at some point they upgraded it to act as a DMR and never fully changed it back?