I'm convinced I want one of these for my tool kit, but I'm going to want to do a practice run on an old wheel before I get stuck on the road. Does the kit come with only enough parts for one repair?

I have one.
The kit can repair one spoke,but it's reuseable.

I have one and ended up borrowing it to a friend when he popped a spoke. I wouldn't want to do a century that way, but it got him home. I'm a believer.

Yes, the idea is that the repair will get you to the next bike shop where you can get a regular spoke of the right type/length. Then you replace the Fiberfix spoke and it's ready to use again if you have another spoke break.

AND...you needn't remove your cassette to repair a broken rear drive- side spoke.

So the spoke doesn't stretch too far to be reliable when it's re-used?

Also, can i buy one in a bike shop, or do I have to order online?

It's basically just a very long piece of kevlar. You don't need to worry about it stretching.

Some bike shops stock them, some don't. You'll just have to ask locally.