The Woosh Bikes website should be your first port of call - they have a wide variety of kits to choose from (both fully and not-quite legal) including hub and crank drive jobbies. Their pre- and post-sale support is fantastic - they really go the extra mile to make sure that you get all the right bits and then help if you have any questions about fitting. Al the documentation is on-line, so you can see exactly what you will be getting before you commit.
There are, of course, cheaper alternatives out there, but with those you need to be sure that the battery is built with decent cells, the connectors are waterproof (and foolproof!) and that they can help with the silly things like the pedal sensor won't quite fit or you need different sensors for hydraulic brakes.