AEG is your best bet since it's the cheapest way to go about it.
If you just want something for the look then grab yourself a clear Aftermath Kirenex and paint it if you so choose. Otherwise if its going to also be gamed at minimum I'd suggest a Jing Gong AR.
Otherwise you're probably looking for a "DEWAT" (Deactivated War Trophy) AR, but those aren't cheap since they're real steel, just deactivated with rivets and welds and such.
ಠ_ಠLess QQ more Pew Pew
READY TO >> RACE