I am looking for recommendations for US made parts in order to have a G3 built on a JLD receiver and trunion.

I would like to avoid fitting the rifle with cheap aftermarket stock parts, and I would like to use magazines in thier original configuration, without having to replace floorplates and followers.

Does anybody have a good source of US manufactured replacement parts to bring a semi-auto G3 build's US parts count up to par?

Try www.jidenter.com/parts.htm  

Timdawgs got it.

JLD currently makes more high quality US compliance
parts for the G3 than anyone.

If your using a JLD receiver and trunnion,that leaves
5 more US parts you need if I remember correctly.
(That's with a muzzle device)

I recommend the following:
JLD heavy barrel .Some people prefer the lighter original barrels .
JLD:Hammer
JLD :Sear
JLD:Pistol grip
JLD:Cocking handle.These are wider and slightly longer than the originals.
They are easier on the hands.
Investment Grade Firearms :Flash hider or muzzle brake.

These are just what I personnally have experience with.

I highly recommend avoiding depending upon magazine parts
for your compliance parts.
Their availability is off and on ,and G3 mag prices are at an all time low.
You'll save money over all depending on permanent parts on the
weapon for your compliance parts.