You can get the official BOSCH seal kits from a Bosch supplier for under $15.
The kit is DGK-126 for a NA pump.
If you are doing a legit reseal, you will also need the main input shaft seal, which doesn't come in the kit and is actually more expensive than the entire kit by itself. (Around $20 dollars)
Part number 1 460 283 312
If you want to be real cheap, you could just order the few little O-rings and seals for the throttle shaft, lid, and timing piston covers individually all for just a few dollars.
Really it's just how much of a reseal do you really want to do and how far do you want to dig into the pump.
It's a gamble though cracking open a pump and only replacing a few seals, I wouldn't recommend it, but if the pump is dry and not weeping anywhere beforehand you might get lucky.
These pumps are pushing 25-30-even 40 years old now so the seals you don't replace may or may not decide to cooperate with you after you go disturbing things.
I'm not trying to fear-monger here, just want you to be aware of the possible side effect when taking these old pumps apart.
My experience on a '92 pump with about 360k was if you crack one of these open and try to band-aid a seal or two, the thing will just start leaking from somewhere else soon after and I regretted not doing a full reseal the first time I had it off the car and semi-apart already.
But hey I didn't really know any better and was looking for a quick, cheap fix to my leaking throttle shaft (after doing the gov mod of course).
Well that turned into a nightmare of chasing leaks all over that damn pump for the next month and getting low 20 MPG's till I ponied up and bought the DVD on eBay for $30 and all the seals for another $35 and did a complete reseal.
It hasn't leaked a drop now in over 40k miles now after doing it the right way.
The downside is you will never get it
exactly back to how it was beforehand so will take some tinkering, but you can get it close enough if you pay attention.
The shops will gladly take your 6-700 dollars and do it for you though if you wish.
There are tons of diesel shops / Bosch suppliers that will happily order these parts for you and even drop ship them to you.
You can get any Bosch part ever made, seals, nozzles, internal pump parts, anything really from a Bosch supplier.
These are the two local places I have used and they are very friendly and helpful, even let me go in the shop area to watch my injectors being calibrated and tested and talk with the guy.
Thinking about getting a turbo pump officially calibrated on the dyno with them too later this year, since I already resealed it, it wouldn't cost as much they said to just get it calibrated and maybe tweaked a little how I want it. (Timing advance, fuel) Most of them have driven or drive VW diesels as well all throughout the years.
http://www.mddistributors.com/http://theaustinfuelinjection.com/Here's two places in Virginia that look like they could help you out, give them a shot and see what they say.
https://www.commonwealthdiesel.com/http://www.blueridgediesel.com/