You don't need a hot-shoe flash. What you need a flash that has a PC connection. All cold shoe flashes will have that, and lots of hot-shoe flashes do too -- but not all. For those that don't, you'll need a PC adapter -- so you probably want to avoid that.
The other thing you need to decide is if you want a manual-exposure only flash, an auto-exposure only flash, or one that has both. You can get one that has both (more flexibility) for the same price as the others.
Third is the power. This is pretty much dependent on the number of batteries it holds. A one AA battery flash will be small but less powerful. A four battery flash will be larger and heavier and have lots of flash power. As power increases, the price generally increases.
There are a ton to choose from. No need to buy new. Here's just what ONE flash maker offered -- Minolta:
http://www.subclub.org/minman/flashes.htm