The champion software for this at the time was Ultimatte Advant-Edge. Comes with a USB dongle key. Maybe you can find a used copy.
But seriously, in all the years I did green screening in FCP 6 and 7, (and I still do) the built-in chromakeyer did just fine, PROVIDING the footage was well-shot and providing that before you try the key, you first adjust all the levels in the raw footage. I never shoot greenscreen without also shooting a chip chart thru the lens on that set in the same lighting conditions. Then in post, first I color correct to those bars or Macbeth chips, and BAM; the chromakey is 98 to 100 percent perfect right from the start. And don’t forget, you can invoke multiple instance of the keyer. Adding the other enhancement tools like spill killer adds the finishing touches. I like to sometimes generate a drop shadow and color it the same color as the talent or same color as the CK background. It can sometimes help with edge problems.
An old trick I used was to replace as much of a bad green screen as possible with a fresh, NLE-generated green, using crop tools and masking. Even turned some green screen to blue screen, before trying to make the key happen. You can use the 3 way color corrector to tweak unnatural colors just to make a clean matte. Then export the matte, re-import it in a “sandwich” between the re-corrected original and the final composite, and you’re good to go.