That varies a bit. For mail order, ecost.com and buy.com are usually pretty good. Amazon.com isn't bad sometimes. If you do your homework on pricing and know exactly what you want and what you want to pay for it, then eBay is a good choice. Some of the reviews here at PDABuyersGuide, like my case reviews, have links to good merchants.
Sounds like you have your bases covered on the essential accessories. The only thing that you didn't mention was an SD card. Get the fastest, largest card you can afford. Again, check out the reviews here.