There's a IR triggered device called "The Lightning Trigger". It's a rather expensive ($150 or so) IR triggered device that hooks in through a cameras electronic shutter release port. It doesn't work with the older manual cameras that I'm aware of.
Any flash of light in it's reception frequency will trigger it. I don't remember the specs on the frequency it triggers from, but I do know that my TV remote would set it off. Add to this lightning flashes, so it had to have a pretty broad range. The problem I had with it, is just that. Dang near anything would kick it off. Flashes from behind, or flashes outside the view area, etc. I've still got it, but it hasn't been used in quite some time. I keep it around in case I need to set up an IR trigger for something.
Your best bet is to start out with print film and do like Charles does. Print film is considerably cheaper. Have only the film developed and no prints, until you have one you want to keep. Use ASA 100 speed film. Use long exposures and a tripod (or something you can mount to your vehicle if you don't want to get out).
Take a look at where the lightning is coming from. Where it's striking. Is there a pattern, is there a concentrated area? Frame your shot, go with an aperture of about f5.6 (Less if your lens will do it), use your shutter release and have at it. I would start out with about 1 minute (night time) exposures up to about 3 or 4 minutes. If you get a good flash, move to the next frame.
Check out the web. There's plenty of resources out there to give you a good idea of how to proceed.
Oh yeah, you CAN do digital. It's a lot harder and more expensive to start up with, but you will eventually get enough back in film/processing savings to make up for it.