I've got a toilet with a "pressure-assist" tank. Instead of just
water filling the tank, there's a self-contained unit in the tank.
It's been leaking from the actual flushing mechanism.

It seems I need to replace the flush cartridge. I've unscrewed it and
examined it and can't find any leaks, but I'm not sure what else to do
that replace it.

Does anyone have experience with these cartridges? I went to the
local plumbling store and to HD, and both looked at me like I was
crazy.

So I want to order a cartridge off the internet, but don't know if
they are universal or interchangeable or what.

On the inside of the tank is stamped:
American Standard
4086 Tank 2
M93

The tank inside the tank has a model number M-101, Flushometer Tank C
285247.

I guess I'm asking which cartridge I can order. Are they universal?

Thanks