Regarding the store, Adorama and B&H are generally excellent, with free shipping in the US (and buying from them, you can support this forum
)
With your budget, the best-featured flash you can get is probably the Sigma 530 Super (or the 610 super, if it's available now). The Standard version has less features, but is also excellent and inexpensive.
Metz flashes are more compact, similarly specked, but with more options (like a USB port for firmware updates). Pentax flashes are excellent but cost a lot more.
It really depends on the features you want. If you plan on using your flash in automatic mode, like you would do the built-in flash, you need PTTL. With that mode, the flash and camera talk to each other and set the exposure together.
If you plan on doing things manually, you could get a much cheaper flash like a Yongnuo 460 that you can buy for 40$ shipped to anywhere in the world (dealextreme.com).
A thing that I would never live without is the tilt and swivel function for the head. The Pentax AF360 is an excellent flash, with a tilting head, but it does not swivel, a deal breaker for me. You will regret getting a flash with no swiveling head.
For the flashes I recommended from Sigma, the Super version gives you PTTL and fine tuning of manual controls. The Standard version gives you PTTL and limited manual controls.
apart from that, it boils down to the features you want:
Zoom head (with or without controls)
Tilt
Swivel
PTTL or manual only (or auto, if you use a used flash)
AF assist beam
Flash power
Flash recycle time
Battery life
Weigth
That's about it.