If you want to buy new, stick to BHphotovideo. If you want to buy used be careful. Probably stick to craigslist or ebay. Look for one with less than 50 hours on it. Less the better.
For accessories, traveling i highly recommend the Portabrace bags. They're very durable, lightweight and easy to carry. I have a pelican case for my XHA1, and while its unbelievably durable, it's impossible to carry anywhere. If you want a shotgun style mic, the Rode NTG-2 is one of (if not the most) popular midrange mic for the DVX100B and similar cameras. Definitely buy a few extra long lasting batteries. (If buying used, it often comes with a few) If your buying new, be sure to buy some lens filters to keep on the front of the camera to protect the lens. You want a 72mm set of Multicoated filters. Probably a UV and a Polarizing filter at least. These run about 30 dollars for a set of 3 assorted or so.
I'm sure i missed a few things, but that's a good start.