The FBI doesn't want you to know this, but as a taxpayer, everything in their arms rooms is technically yours. You can walk right in and ask for the spare parts!
Or if you can find the selector and housing, GSC has the trigger pack itself:
https://www.gatewoodsupplyco.com/HK-Ambi-FBI-Semi-Auto-Trigger-Pack--9mm-_p_564.html
You can also look for someone selling an SP5 takeoff housing and drill the front takedown pin holes yourself.