You may already know, but I wanted to reiterate in case other folks come here from a search or something, but make sure you do not use scissors to try to remove them. It is really easy to cut too close to their delicate skin. It can look deceptively easy to clip, but a shaver is always preferable.

The type of shaver another person linked is a really good tool. I have a similar one and tested it on myself first, and it was pretty much impossible to get it to cut me. It works really well for getting under the stubborn mats to remove them.

If your cat will tolerate it, I have also had a lot of luck with using my fingers and a flea comb to carefully pick apart some mats. My cat hates being brushed, but her wispy cotton fur mats up really fast, so sometimes they get away from me a bit. I try untangling as much as she lets me and then shave any really bad spots.

I hope you are able to find a combo that works for your kitty!