Do you know what part of your tank the mantis lives in. One of the more successeful stories I have heard was when the people just remove the rocks in the area of the mantis and just looking for him. Mantis shrimp are very good hiders and are fast so you have to have a good eye when your removing the rocks.
If I catch him you can have him. He moves around the tank. I have seen him everywhere. The rocks he is in are usually low in the tank and he is gone before I get to him. At his rate of growth I expect that I will be able to snag him within two years when he is lobster sized.