Get any 1970s-or-later electronic flash. One thing to look out for is having a flash with an automatic flash mode. Manual flash is bothersome because you need to set aperture on the camera in proportion to the subject distance, while with automatic flash you set film sensitivity and aperture on the flash unit and it automatically regulates the brighness of the flash using a small sensor built into the flash unit - then you can just set the flash unit, say, to f/4, set the same aperture on the lens, and flash away without bothering about exposure at all, and it's surprisingly accurate (better than in-camera TTL in some situations). A flash cord is nice as well if you need the camera's shoe for something else, as the others have pointed out, but there are external hot shoe blocks with a cord attached to them which you can get for $1 on eBay, so this is not a must.
Myself, I'm using a Metz 32CT3 which has a manual mode, three automatic flash modes for different apertures, is comfortable and easy to use, uses standard AA batteries, can be used with the standard SCA system of flash accessories so you can use it with more modern SLRs as well, and works with a flash cord if you get either the right SCA foot or a simple hot shoe block with a cord. I've been using that with a Bessa R for quite some time, and the flash set me back 25 EUR. So you don't have to spend a lot of money.
Philipp
