It's been a couple years since I bought mine and to my knowledge the 12ga units have been discontinued. Only place I know of that still has 'em is The Country Shed. I've bought a ton of stuff from them and they're top notch. The 12ga UTG rail (item# MNT-HGMB500A) is a ways down the page and is the one I have on my 590a1...
UTG 12ga Mossberg 500 Forearm
UTG 12ga Mossberg 500 Forearm