Any lower will work, some 22 uppers don't like notched hammers some it doesn't matter. It basically all the variation intolerances, a few thousandths here and there can stack to cause an issue with notched hammers. I've used DPMS and RRA lower parts kits without issue. I have used Palmetto State Armory, RRA, Double Star, DPMS, CMMG and Colt lowers with with 22 uppers and never had a problem. Basically any AR-15 lower will work, even te polymer lowers like Calvary Arms, New Frontier Arms or ATI Omni. It is pretty easy to assemble a lower or you can buy a complete rifle in 22 LR from CMMG.
I wouldn't buy a complete 22 LR AR-15 from American Tactical Imports (ATI) because I don't care for their uppers. If I were going to buy a new 22 LR AR-15 upper I'd probably buy a CMMG, TAACOM, Tactical Solutions or DPMS depending on which had the features and price I liked. I've read good reports on the Core 15 but Inave never seen one in person, nor have I seen a Nordic.