SPR build need to keep the cost down

Hey guys. im trying to build an SPR. Im basing it around a Stag Lower, an YMH upper, and a DPMS MK12 18" barrel. Im trying to use a magpul CTR stock and some sort of quad rail full length free float. My main problem at the point it keeping the cost down. Right now im hovering around 910 with everything. Does anyone know where i can get parts on the cheap, or would anyone have any parts they want to sell?

Thanks

Special Purpose Rifle. An Ar15 variant. Google Mk12 SPR. i need gas tube, gas block. Back up irons. a bolt. the whole buffer assembly and a free floated rifle length quad rail. But really im open to any ar15 parts besides a non rail free float. PM me if you have offers