Get the Adams Arms while you still can. 5.45 ARs kind of died out when ammo got cheap a few years ago, and it hasn't been long enough for that production to recover.

There was only a few manufacturers/sellers anyway, but maybe this list will help you in your internet searches:

Smith & Wesson
Spike's Tactical
Ballistic Advantage
Adams Arms
Model 1 Sales
Voodoo Innovations
M&A Parts
Faxon Firearms