The problem with that part is it is not a simple oring. Don't go by what the illustration shows. It is a seal bonded to a metal shim under the cover. If you look at the cover you will see to tabs bent over. That is the locating tabs for the shim/seal. The aftermarket kits will not have that part in it. it is still available from GM under part number 7817486. $38.44 list price. It contains the shim/seal and four new bolts. There are two ways to replace it. Center the steering gear , pull off pitman arm, remove bolts, pull cover and sector out. Replace seal, reinstall in reverse. The only problem with that method is you may disturb the bottom shaft seal and create a leak. If you are not going to replace the sector seal, you can just pull off the top cover. You will have to unscrew the sector adjustment to do that. You can do the readjustment yourself. I would advise to do some research on that if you don't know how because you will either cause a bind or have to much play in the steering if not done properly.
Google GM7817486 and then click on image. You will see what I'm talking about