Not sure exactly what type of external mic you were after, but I'd look at the Audio-Technica ATR-3350 if you want a cheap, but decent lavaliar mic.
It's about $25-ish. It's a little soft when you record, so you have to bump up the volume in editing and for the price you won't really find a better external lavaliar without spending a little more money.
Another option is the Sony ECM DS70P, on ebay for about 15 bucks (last time I searched). Assuming it's the same 3.5mm, here's a video of a generic version of it in use that you can find on ebay for around 8 bucks.