I use a Super Reef Octopus and have had good luck with it. It is an old design. But a protein skimmer is just a tube with a bubble maker at the bottom. So it isn't exactly high tech. Having a good pump is a good idea. Also having a decent adjustment mechanism is important. Skimmers can be finicky so being able to dial them in is good. Also it is good to oversize your system for your system, the manufacturers tend to over estimate.
Bulk Reef Supply and Marine Depot both carry a bunch of skimmers. They give ratings, dimensions and important specs. It is a good place to start.
One thing to be careful about is to be sure that you can actually get the cup out so you can clean it out. Often the over head space in the cabinet is limited.