I bought Caleb's collar from Pawmark http://www.pawmark.com/index.php . Awesome customer service! Not only are they fast, but, with this collar, they sent me the wrong one. When I emailed them about it, they said to keep the one they sent, and they would send me the correct one. I had the right one within a week.
Dog.com has good prices, but they're shipping charges have stopped me from ordering from them. Same with Drs Foster & Smith- I've bought a few things from them years ago, but their shipping fees are kind of high.