I just picked up a couple from GlockParts.com last week, 13 rounders.
Last I saw MidwayUSA had them on sale but ofcourse they didnt have them in stock. What good is a sale if you dont have any in stock, lol.
The G21SF takes the same mags as the G21. I ordered an extra 13-round mag from topglock.com (although it was backordered when I ordered it) and it works perfectly in my G21SF.
From the time of my order to the time I received it, it took about 2 weeks, but that mag was the only thing holding up my order. Once it shipped, I received it promptly!