They are a pain to catch. I took at eveything in my 110G and took around a hour two catch my 2 damsels.
You can buy fish traps or try making one youself. 2 methods I know of to make a trap
1. Get a piece of PVC
pipe about 8 inches long or so and large enough for the fish to swim in. Drill a few holes in the side and cap one end of the pipe. Tie a string on the open end and put some food inside the pipe. Lay the pipe on the bottom of your aquarium and when the fish goes in to get the food quickly pull up on the pipe hopefully trapping the fish inside.
1. Get a plastic gallon or 2 liter botte and cut the top 1/3 off. Flip the top around so the little opening is inside the container and put food on the inside. When the fish swims in the little hole pull the container out.
Both will require some patience and the fish may not go in them right away.
Good luck :P