I agree that doing it yourself is the way to go. It's not that difficult and you'll probably be doing it again (and again and again...). I think the plastic welder Don posted is a good bet if you don't already have something. It's best to get plastic from the same type of boat that you're welding if possible.
From what I've seen, if you have a small crack your chances are really good of getting a weld to hold. If it's a large displaced crack under the seat it's going to be more difficult. If it's not under the seat you're in luck.
It also depends on why your boat cracked. I've found that often once you get a crack from wear and tear it's an indication that your plastic is done worn out and you're about to get more all over the place. I've chased small cracks around my boat for months and found that the welds almost never open up again.
I've taken to doing a weld on the outside, then on the inside, and then slapping on a few layers of Gorilla tape on the inside (just in case). It's not as good as new, but pretty bomber. I would perform this in a well ventilated area.
A lot of people end up using a welded boat for roadside runs and keep an uncracked one for runs that would be painful to have to walk out of.
deepsouth and I were once discussing putting together our own amateur "how-to" welding video but have been too lazy or apathetic to do it. I'm thinking a special feature on the new HH website