M4 Or Nova Magazine Caps

[M4Madness]

Assuming that they have the same thread pattern, where can I find one cheap? I've seen the Nova cap at Brownell's for around $20, but figured I'd see if I could find it cheaper somewhere else first. I've got another idea running through my head, but don't want to ruin my only M4 magazine cap.

[StarLynx]

I was do'n a search for a rear aperture replacemnt for my M3...Ghost ring....& came across a forum that somebody or dealer had four of those forsale..Been go'n thru my history..but haven't found it yet....I think it was a rinky dink Tactical SG Forum......Hope it helps..