I use a Audio Technica ATM 35 and I'm very happy with that.
There's also a cheaper version from almost the same mike from Samson.
At link#2 you see a small aluminium strip to put the microphone on.
I used the old fashioned snap-on for years, until it was stolen in a home burglary. Someone gave me a condenser mic which I have now used for some years. I am about to order the big snap-on again. I'm going to check Savoy Music first--they probably have some. Or any Louisiana accordion builder will probably have them.
One thing to remember if you go with a smaller condenser mic, is that it needs phantom power. So you either need to plug into a board that has it, or carry a phantom power box with you. Recently I bought a new powered speaker that I can plug directly into, but didn't realize till my first gig with it that it didn't have phantom power.
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.