You can save a ton of bucks by making your own, though. I made like 25 of them out of a couple yards of felt from Hobby Lobby (had my mom sew them, though). Cost a total of like $15, including the cost of some nylon thread.
If you go this route, you need to have a 'business name'
Look at the ones titled "Heat-Welded Polyester Felt Filter Bags" I know some reefers locally that swear by them. You can get them in as low as 1 micron... they use 100 and say their water is SPARKLY and wouldn't go lower.
I don't have a business name and I order from them--usually gets delivered the next day. Maybe it's on a state-by-state basis or some strangeness.
At any rate, I've been using the Heat-Welded ones for years in the 4"x6" size. Never had one fail and they go through a lot of washing before I decide the material is too pilled and pitch them. I use 200's normally, and change them every other day. Once a month or so I run a set of the 100's, but the turtle is too messy to do that continuously.
The heat welded ones have a plastic ring on top, that in most cases provides enough support and sealing, especially if the return is directly into the sock itself. The ones with a seal are better for indirect flow, such as you find on the sumps like the aqueon pro flex line. However, I use both types on my proflex 4 without much issue. I can take pictures when I'm at home later if you need them.