Canik TTI Combat (Smoke), which TTI Base Plates Work?

I just picked up my TTI Combat (smoke). My first Canik. Really liking this stock trigger. Anyway, I wanted clarification as to which exact TTI base plate extensions will work with the standard CaniK TTI combat 18rd magazines. I emailed TTI directly and they advised they do not sell the +3rd extensions that come with the TTI Combat. Do they make any +3rd’s that will fit a standard Canik TTI Combat 18rd magazine?

I’ve tried to find the OEM Canik TTI Combat magazines (21rd) with the black base plate, but they’re difficult to find anywhere. Also, why are they so expensive?!