You can buy the stamped receivers online or through a gun store. I've seen them at shows but it's hit and miss.
The flat receivers are not considered guns until you bend them, which is another complication of going this route. You have to register as soon as it's bent
You don't have to have a US trigger group. You do have to be compliant with federal law 922r. which states that no more than 10 parts can be imported on the gun. Using a domestic trigger group is popular because it counts as three compliance parts (trigger, hammer, sear). The US triggers are generally an improvement as well.
I don't know if you can post links here, but there is a website called AR15.com. The AK section is probably the best source on the planet for all the legal and technical help you'd need. Here is the actual text of 922r. Be 100% sure you understand it before you put anything together...
The Imported Parts Law (1990)
(178.39 otherwise known as 922(r) 10 Foreign parts law on semiauto Rifles & Shotguns)
Sec. 178.39 Assembly of semiautomatic rifles or shotguns.
(a) No person shall assemble a semiautomatic rifle or any shotgun using more than 10 of the imported parts listed in paragraph (c) of this section if the assembled firearm is prohibited from importation under section 925(d)(3) as not being particularly suitable for or readily adaptable to sporting purposes.
(b) The provisions of this section shall not apply to:
(1) The assembly of such rifle or shotgun for sale or distribution by a licensed manufacturer to the United States or any department or agency thereof or to any State or any department, agency, or political subdivision thereof; or
(2) The assembly of such rifle or shotgun for the purposes of testing or experimentation authorized by the Director under the provisions of Sec. 178.151; or
(3) The repair of any rifle or shotgun which had been imported into or assembled in the United States prior to November 30, 1990, or the replacement of any part of such firearm.
(c) For purposes of this section, the term imported parts are:
(1) Frames, receivers, receiver castings, forgings or stampings
(2) Barrels
(3) Barrel extensions
(4) Mounting blocks (trunions)
(5) Muzzle attachments
(6) Bolts
(7) Bolt carriers
(8) Operating rods
(9) Gas pistons
(10) Trigger housings
(11) Triggers
(12) Hammers
(13) Sears
(14) Disconnectors
(15) Buttstocks
(16) Pistol grips
(17) Forearms, handguards
(18) Magazine bodies
(19) Followers
(20) Floorplates