It is pretty much like flash drives these days; unless you're looking for special features, most are the same.
For the most part, just find a cheap one that has okay reviews, and go for it. I like
Newegg.com - Computer Parts, PC Components, Laptop Computers, Digital Cameras and more! myself. Find a BT adapter that has plenty of good user reviews, and just buy it. Probably shouldn't cost you more than $15!
I have one but don't use it anymore. I prefer to use a USB cable to connect my phones nowadays. The latest is my iPhone. I could use BT to wirelessly connect (I think, not sure if iTunes can handle that - I know before my Nokia Series 60 phones had no problem with BT, even IrDA).