Crawfish are found all over the United States, from Alaska to Florida. The best fishing grounds for them are in the American South, but there's amazing lakes and rivers full of them in California and New York too. Crawfish are also eaten in Europe, South America, Australia, Asia and beyond. Although they're not a staple of food for most Americans, they could be. Their popularity in Louisiana has more to do with local's love for them rather than some presumed natural abundance.
That said, the waters in Louisiana are full of them. Making a trap to catch crawfish is relatively simple. Mine is made from wire mesh of a small gauge that I dumpstered from a construction site. Look around your town and see if anyone's building a stucco structure. Odds are they'll be throwing out wire mesh used as backing for the stucco they apply. This is really ideal material for making a trap.
You'll also need some wire you can use to twist-tie fasten the mesh together with. I got mine by pulling out the copper wire from inside an old electrical cord and making litle ties out of it. Whatever works. Having some paper, pens, good wire cutters, etc. helps a lot.
First you cut out a big rectangular piece and bend it into the shape of a big cylinder. The ideal size is maybe a foot and half in diameter and a few feet long. Next you need to cut to large half circles out of the wire mesh. These will be the ends of the trap that allow the crawfish in, but don't let them out. I recommend actually cutting out a few different sizes of paper half circles first, folding them on themselves (so that the flat edge meets and you have a perfectly shaped cone) and then seeing how well these fit in your cylinder's ends. Once you have a good size paper cut out use it to trace-cut your wire mesh half circles.
Tie the cones together and cut a small opening at each one's tip, about 2 inches in diameter. Then lower them, pointed side first, into the cylender's ends and fasten them well. Now you have the basic trap, all you need is an access door on the side (a place to insert bait and take out the catch). Cut a hole in the side (whatever shape you like) and then cut out a larger piece of wire mesh from the remaining stock you have. Put this larger piece over the whole and find a way to fasten it down like a door with little hinges on one side and a lock on the other. I simply used little pieces of metal twisted into a circle to secure the door on one side and I tie it tightly shut with a little piece of string.
That's it. Now it's ready to go fishing. The best bait apparently is fresh fish parts but chicken works, as do other kinds of meat. It's best fresh, not rotten. Put the bait in the trap and lower it in a creek, bayou, river, lake, pond, etc. Leave it overnight as crawfish are nocturnal. Hopefully when you pluck it out the next day there will be some crawfish in there.
Best way to eat them is to boil them with lots of salt and hot pepper.