You're right, the pack holders are useless. Does it already have a ground-glass back?
9x12 film holders are a bit of a complex world, because there are a bunch of different standards to which they were built. Your best bet may be to find out the Zeiss part number---from googling a bit I think it MAY be 665/7, but I wouldn't bet the farm on that. (Note that Zeiss made several different forms of 9x12 holders, I think because they bought up several smaller companies whose cameras were heterogeneous.)
Rollfilm backs exist, but again you have to find one to fit a Maximar, the backs tend to be expensive, and you only get 6x9 or smaller formats---I think it's more worthwhile to shoot 9x12. Fomapan 100 is fairly easy to find in 9x12 (Freestyle routinely carries it), a few other films are sold in that format in Europe, and I believe the recently released Adox CHS II 100 is supposed to come in it eventually.
I've gotten good service from Pacific Rim Camera; what they will and won't have is very random, but they're a reliable seller and I've always found their descriptions to be accurate. eBay is hit-and-miss as with other things---sometimes you get a bunch of holders in great shape, sometimes they're masses of rust or riddled with light leaks or the wrong fit altogether. Often the best way to get holders is to buy a camera that comes with them.
Shooting large format involves a lot of steps, but none of them are particularly hard. Getting the hang of loading the holders in the dark is the hardest part, I think. Other than that:
1. Point camera at subject
2. Put on ground-glass back
3. Open shutter
4. Disappear under darkcloth and look at ground glass
5. Emerge to tell people to stop laughing at you
6. Go back under and focus until the image looks good
7. Close shutter
8. Try to remove ground-glass back
9. Curse at sticky edges on ground-glass back
10. Pull hard enough that the tripod moves
11. Go back to step 1 and repeat until you get the back off without moving the camera
12. Check that you remembered to close the shutter
13. Insert film holder
14. Set aperture
15. Check that you set the aperture
16. Check again
17. Set shutter speed
18. Dry-fire the shutter to make sure everything is working
19. Make exposure
20. Realize that you forgot to pull the darkslide
21. Pull the darkslide
22. Check that the lens is still stopped down
23. Make exposure for real this time
24. Realize that even with all those checks you STILL forgot to stop the lens down
25. Curse
26. Reinsert darkslide and remove holder
27. Try another holder
28. Remember all the steps this time
29. Explain to spouse, friend, parent, etc. why all this is worth it
30. Tell them again to stop laughing
31. Go home and develop
32. Get a fingerprint in fixer on the negative
33. Curse some more (try another language for variety)
34. Take up painting instead
At least that's my general workflow, but everyone has their own preferences. Don't forget that there's some sort of rule that forces you to wear a cowboy hat when shooting LF landscapes.
-NT