Kragen/Oreiley's/Shuck's carries the complete seal kits with piston rings, etc. It's about $35-$40.
Napa does carry them too, but their kits don't come with the sector shaft cap seal for some strange reason. They're also about $15 more. They apparently do not even carry that seal for some strange reason...gets frustrating when I see Napa's quality going downhill like that.
You could replace those seals without pulling the box if you really wanted to. I'd pull the inner fender and the tire off. Then, unscrew the 4 9/16" bolts on the corner of the sector shaft cap. Leave the 5/8" nylock in the center alone, that adjusts the preload on the sector shaft bearing. Once you get the 4 bolts off, tap the bottom of the sector shaft with something like a dead blow or rubber mallet while wiggling it. It will start to push that aluminum cap up and off of the top of the box, you just have to overcome the friction of the seals on the bottom of the box. Once you get the sector shaft out, remove the circle clip and use a flathead screwdriver to pry the two seals out of the bottom. There are two washers in there too (the seal kit should have a half-assed exploded view of the box in it).
Use a socket and tap the new seals in (make sure you orient them correctly and put the washers in right) after you lube everything with some PS fluid. You may want to wrap the very bottom of the splines of the sector shaft with masking tape to keep from damaging the new seals if you can't see very well, but make sure you don't go too far up the shaft and don't wrap it more than once or sometimes it will get stuck because the seals fit really tight.
Rebuilding the whole box really isn't even all that difficult.
There is a seal and dust shield on the input shaft, there are 2 seals (single lip AND a double lip) on the sector shaft, then there is also an O-ring on the top cover plate for the sector shaft (some use just an O-ring while others are a steel shim with an O-ring attached, either is correct and work). The sector shaft seals you'll probably need to remove the sector shaft and use some form of driver to remove the sector shaft bearing and both seals and spacers at the same time as the seals together are probably about 3/4"-7/8" thick when stacked and it's doubtful you'll be able to pry the inner most seal out.
You don't need to mess with the bearing to do the seals. Just need to pry it one one side, then the other, go around in a circle, etc like doing hub seals. Might need to tap on the screwdriver lightly with a small hammer. Just be careful and work slowly. I wouldn't try to drive everything out at once because you would be hitting it so hard, I think the very thin bearing race would get destroyed and possibly chew up the housing. Those seals have a lot of friction, as does that bearing that's about an inch long. I haven't heard good things about rebuilt boxes, so I always just rebuild them myself because it only takes a couple hours and is under $50 if you don't need new bearings (haven't had one yet that needed them, and I've heard most of the remans don't have new bearings anyway).
I think Napa may have carried a seperate main seal kit for like $15 that had the shaft seals...that may have had a cap seal too, but I'm thinking back to something the parts guy showed me a long time ago when I was trying to get the master seal kit. Pretty sure there are main seal kits available, which should have the three seals we're all talking about and probably a new o-ring for the piston cap.
Two very helpful writeups:
http://www.pirate4x4.com/forum/showthread.php?t=570869
http://www.fourwheeler.com/techarticles/drivetrain/129_0801_saginaw_steering_box_rebuild/index.html
