Did only first gear get trashed, or did the corresponding section of the cluster gear get eaten, too? If only one is obviously broken, you need to check the other closely to make sure it's truly undamaged. If it's OK, you can replace the one gear, but it may be a little noisy. Low gear is usually a bit noisy anyway, so I wouldn't be too worried about it.
A transmission place is liable to stick you for a pretty good labor charge, but if you're adventurous and have access to a good lock ring tool, you can probably do it yourself. It's a bit intimidating, but if you take it apart carefully, keeping everything in order, and noting which way everything faces, you should be fine.
has rebuild kits and may be able to supply used gears if available. They also have ID info. Their kits may also include diagrams, but I don't know for sure.