Get an RT Pro ULE with Xvalve, this is cocker threaded.
Automags.org is a good place to start in the buy sell trade, or here on PBN.
You can get a new one for around 550
So a used one 350 to 450.
"When the people fear their government, there is tyranny; when the government fears the people, there is liberty. When injustice becomes law, resistance becomes duty." Thomas Jefferson
"Political power grows out of the barrel of a gun." Mao Zedong