You can buy completed receivers from a lot of places. I recommend Childers' Guns. Childers can custom engrave your serial number, trust name, or whatever you want. Just make sure you tell whoever you buy from that it's a Polish underfolder.
You need a standard, Soviet AKM pattern barrel. Green Mountain makes a chrome lined, parkerized, threaded barrel with the handguard retaining slots cut. It's a pretty good deal, and a great barrel.
You'll need 6 US made parts (if you use a threaded barrel). The easiest route would be barrel, receiver, muzzle device (I like the slant brake from akbuilder.com), and a fire control group (hammer, trigger, disconnector) of your choice. The Tapco G2 is the long-standing leader in sales and is a good FCG. Stay away from anything else Tapco. Several others also make a FCG, like Arsenal (kvar.com).
You will need a rivet set for a Polish underfolder. I highly recommend the rivets from akbuilder.com.
So here's your parts checklist:
__ Receiver
__ Barrel
__ Muzzle device
__ FCG
__ Rivet set
In order to build this kit you will need access to a hydraulic press (at least 12 tons), a very nice metalworking drill press, (ideally a milling machine), a fixture for pressing the barrel in the front trunnion, fixtures for populating the barrel, headspace guages, fixtures for installing the rivets, and some various drill bits (or end mills) and a reamer.
The long pole in the tent is drilling the barrel pin channel in the virgin barrel you buy. The pin is .002" wider than the hole. It has to fit tight. The hole has to be drilled precise. If you mess this up you risk the barrel shifting inside the trunnion, and an excessive headspace situation which can result in a kaboom.
Unless you decide to build AKs as a hobby, I recommend finding a pro builder to put it together for you. That would be way cheaper than buying all the tools and risking ruining a part that you'd have to replace. If you're interested in taking up the AK building hobby, head over to www.akfiles.com and start learning.