With kits being relatively cheap and readily available, I'm kicking around the idea of a SAR but in x39. I know I'm  going to need a new bolt, but how would  I go about the barrel?

Probably have the build done by hillbilly or inrange when they start accepting work again.

Turn a .308 or .311 blank, ream the chamber and assemble it. Not sure if the bullet guide will need any notification. As long as the bolt matches the outside dimms you should be ok.

Barrels are not rocket science but difficult to profile without a decent lathe.

AEnemaBay had some 7.62x39 barrels that he turned for sale on gunbroker a while back. He may be able to turn one for you. There have been other discussions and builds on Akfiles for 7.62x39 Galils. The bolt is the easy part, you can use any large stem AKM bolt. I have read different things on the bullet guide. Anything from using the original to using one for an AK. The AK mags will fit with minimal mods.