The standard mic for both internal and external use on a Cajun accordion is the Shure R-65. It is much cheaper than the condensor mics, although bulkier. You can get them from Larry Miller and other builders.
It used to be that the R-65 mic was about the only type you would see in use. What I observed during the 90's was that some players began to experiment with the smaller, but much more expensive (and fragile) condensor mics. But after a while, I noticed a return to the R-65. I asked some players about that, and was told in several instances that even though the condensor had a good clean sound, they were finding that the sound was TOO clean.
I had been considering going to a condensor mic during that time, but didn't jump on it because of the expense. In the end, I decided to skip it entirely, and stick with what I already had. Additional factors that went against the condensor mics was the difficulties in how to clip them on securely (many complain that they don't stay in place very well), and also the thin and fragile wires that seem to break easily. It began to seem that condensors were all around losers.
Following all that, I joined a different band that was all Zydeco and little or no Cajun, so that was then end of using the external mic anyway. I have the same R-65 mics internally in most of my accordions now, except for two in which I installed the mic element of a Shure SM-57 which has a hotter signal.
Thanks David.. Ive heard some similar things....and Im currently using a 57 on the outside and there is some blaze to it. Most in the audience here have little regular exposure to the button box. So I guess I get an easy out there in most cases.
Since I fairly sure my eventual playing will be with an electric band, Im starting to figure an internal mic is best if not to just be able to isolate the sound....
The choice between internal and external mics depends on the type of sound you want. Even with the exact same mic, the results are VERY different. If you want to go for a Cajun sound, it is best to have the mic on the outside. This gives a nice clean sound. But for zydeco, it is better to put the mic on the inside. This creates that cool muffled sound that we associate with zydeco greats like Keith Frank(the sound is somewhat like what a blues harp player gets).
On my D accordion, I have the option of switching between internal and external with the flip of a switch. I suppose I could even use both at the same time, but I haven't tried that. Lately, I find that I just leave the external mic at home.