If you need something in a hurry, just order a stripped lower plus parts kit from a vendor here, then order a complete upper from a vendor that has what you want in stock and ready to ship. Otherwise, just order the damn thing from RRA and wait.
On edit: RRA is known for it's high quality. Everybody wants one which is probably why they are so busy. You could go with a different company, as you stated, but I doubt you will find one as dedicated to quality and outstanding customer service than RRA. This is my own opinion based on my first hand experience with 5 RRA rifles and in speaking with their customer service representatives.
I also don't see any other manufacturer take as much interest in speaking directly to their customers on this site as Steve does. He's here every day to answer our questions and address our concerns. Try posting a question in the Armalite, Bushmaster, or DPMS forums and see how quickly you get a response.