Do you have a 9mm upper for an AR-15?
To get one with mags that aren't custom/proprietary you pretty much have to get a dedicated lower also.
I had a Model 1, 9mm 7.5 incher on my SBR lower, and it was great. All you need is a mag block which tightens in with an Allen wrench & a 9mm hammer (unless your using the CMMG Inc. 9mm upper, which uses regular hammers, allowing a NM trigger if you want it). I shot tons of 5.56 & .22 out of their respective uppers with that hammer in & had no problems.
As for mags their easy to find & cheap.
I believe your thinking of the one lowers that allows you to use Glock mags, but that's the only one that makes you dedicate that lower exclusively for 9mm.
Making an AR into a 9mm is an easy & affordable route if you want a 9mm carbine. I've got one SBR lower that shoots .22LR, 9mm, 5.56 & 7.62x39 all for one $200 tax-stamp.