You need a flash-bracket, and PC->Hotshow adapter. They're pretty standard components and can be picked up really cheaply. Then you can use any hotshoe flash.

Alternativly get hold of a hammerhead flash (which has a built in bracket) like a Metz 45. Either way works, depending on how much power you want.

The only problem is that with the bracket attatched it can be tricky to focus, as the handle gets in the way of the focus control. The easiest fix is to jjust preset your distance, and move yourself to the right distance.