I also use an Audio-Technica clip-on mic. I bought it from Larry Miller (of Iota, LA) way back around 2001 or so. I love it. I don't like the look of the big plate-mounted mic heads like some of the older players have, as I don't want to have to drill any more holes in my accordion, and those large mounts also don't let folks watch the flappers move. BTW, those same AT mics are used by a lot of fiddlers who clip them on their fiddle's tailpiece.
Yes, the Audio-Technica mics need phantom power, but that's not a problem. Most mixers should have phantom power available, although I also carry my own phantom power box in case I need it when playing a gig through someone else's PA in case they don't have phantom power.