The obvious answer is eBay...... I recently got (2) NetGear FA120's from a seller there. They were $20 a piece which is a little pricey, but they were going quick. He had 37 when I did the Buy It Now and a day later he was down to 14.

eBay has all the recommended ones, just use normal eBay buying recommendations so you don't get screwed.

Also I saw a post that Tweeter offers one of the adapters (the FA120, I think) on their website. They are refurbished, but I personally don't see that as an issue.