The frame is the part the ATF usually considers to be the gun - it's the regulated part. You can buy barrels, triggers, hammers, etc., without paperwork, but buying a frame is the key to needing to fill out an ATF form and have the transfer go through a dealer.
I've seen 1911 frames pretty regularly, because lots of people like to do personal builds of The Handgun. You can probably find Glock frames too, but I wouldn't know because I have no use for Glocks. A PT9x series frame is going to be a tough search.