Unicel C-5315 (can find on Amazon and elsewhere) is a better quality, made in USA incidentally, filter cartridge. It is size compatible with the Intex B. As others had suggested, I find the Unicel cartridge does a better job, and also seems easier to clean. Dirt seems to come off more easily when I spray the filter with a garden hose. I always washed and reused the generic B cartridges also - seemed wasteful and unnecessary to throw them away after one use.
Edited to add: If you do decide to buy a Unicel cartridge at some point, double and triple check that you're getting a genuine brand name item. I remember reading lots of complaints from people who ended up with "Unicel compatible" cartridges - i.e. the same lower quality stuff they were aiming to replace.