HKPARTS.com has the complete bolt assembly, but I'm trying to find a stripped carrier to save a few hundred dollars.

Lee Tool and Machine advertises on Sturm full auto German carriers modified for  PTR guns.

Never bought one from him as I have always just welded up my semi carriers.

http://www.sturmgewehr.com/forums/index.php?/topic/2183-%C2%A0ptr-762x39-or-hk-51-stripped-select-fire-bolt-carriers-265-each-delivered/

I don't think its as cheap as a stripped PTR carrier but may be cheaper than a full PTR bolt group.

I had s&h weld mine.

How hard is it to weld up a semi carrier?

I am no welding pro, but filling in that little gap with a TIG is extremely straight-forward.

As everything TIG....most of the work is in the prep.

I first clean and degrease the carrier, then mask off the area to be welded with aluminum tape around the semi cut through slot where its to be welded, then us my blast cabinet to remove the parkerizing in the area to be welded, then meticulously clean the now park free area with a stainless steel brush and acetone or break cleaner to remove any embedded aluminum oxide from the blast cab.

Start a weld puddle in the channel and feed filler rod in until it fills up the ramp area.   Once filled, let it cool down, machine the weld level with the bottom of the carrier and then machine the "ramp" into the front of the weld area.  IF you want hit the weld area with some cold blue to blend it in and you are done.

I only weld the rifle carriers.   For MP5 carrier I high temp silver solder on a  piece of steel as the area where you welding is much thinner, its right over the LP channel and you can tweak the carrier where the LP goes in so it won't fit or turn/rotate correctly with the weld goes too deep.  The rifle carrier are much thicker in the back with just the firing pin channel buried deep in them over the weld area.

That would be the way to go.  They had my SP5K carrier welded and back to me in a week for $80.