Go to an area that has lots of mangroves and mud flats. Go during the lowest tide of the day. You will see thousands of fiddler crabs running into their holes. You can use a shovel to dig them up or you can dig a hole and put a 5 or 10 gallon bucket all the way in the mud. Put any kind of stinky, dead bait in the bucket. Make it so the bucket top is even with the mud's surface. Fiddler crabs will come back out after you back away or leave the area. They will walk over to the bucket and (trying to get to the food) will drop in and won't be able to climb back out. You will have all the fiddler crabs you need to fish.
Another option is to call a local tackle shop and see if they have them. If they do not, see if they are able to order them. They are fairly cheap usually. There are local tackle shops in Tampa Bay that sell them for $5 or $10 for 50 of them. Hope this helps. Thank you for your question!