Any Nikon, Canon, etc flash will work with appropriate radio slaves or PC-sync cables, as will any third-party clones eg Sigma, Metz, etc. Sony/Minolta too, though you require a foot adapter. You should be able to get something with about 80-100Ws of power for under $300 secondhand, under $100 for an off-brand cheapy (Sigma, Vivitar or even a Chinese generic).
When comparing battery hotshoe flashes, the power ratings are very deceptive because they quote GN instead of total energy. And they quote GN with the narrowest possible beam that the flash can produce, which differs between models. So you might find that a cheaper, lower power flash that produces a very narrow beam can quote a higher GN than a more powerful flash that produces a wider beam. My Minolta 5600HS(D) each produce about 1 stop more power than a Sigma flash that claims the same 56m GN.