If you want to catch fiddler crabs yourself look for them on spoil islands and grassy marshes during low tide. Often times these crabs are in groups by the thousands but don't let that fool you into thinking that they are easy to catch!
These little critters are very fast and they hide in small holes burrowed in the sand, so you need to catch them before they get into their hole. If they get in the hole before you catch them, many times you can dig down and get to them.
The easiest way to catch these crabs is to use a castnet with a very small mesh to cast on top of the crabs. A minnow net can also be used to scoop them up but this will take much more time then what the castnet would take.
The male crabs have a big, mean looking claw but there is no need to be afraid of this, while it looks impressive it has very little strength (not enough to even break the skin if it were to pinch you).
If you have some fiddler crabs leftover you can freeze them so you can use them at a later date. Many times it makes no difference if they are alive or frozen the fish still love them.