I wouldnt purchase one...at least to start. I'd make one of those 2 liter soda bottle ones to try first. You just cut the top part of about 3 or 4 inches down so that you get a nice cone shape, then cut off the part that you screw the cap on. Turn the cone part so that when you put it back in, the "point" goes in first. This will leave a wide opening on the outside which gets narrower, and a very narrow opening on the inside, which is harder for a shrimp to get out.
Then you'd put some meaty food, like krill or something in the bottle.
just let that sit for a while. I've seen people saying that works.