Zach,
I had an uncle in the upholstery business and he always used a product called "Dacron". That may be the brand name, but it was a white batting material that was available in thicknesses from 1/4" to 2". Dacron is a non-organic material that is not influenced by moisture, so even if you could still obtain real horsehair, I would not use it.
The PO of my car had the seats done w/o replacing the horsehair, which I think is a mistake because Dacron is cheap and you may as well correct the ills of the past when the seat covers are already off.
There are different types of foam rubber which could be used in the bolster areas, but I think Dacron is what youd be looking for. I don't know where you get it, but do a Google search and I'm sure you'll find a supplier/distributer.
Also, while you have the seats apart, pay attention to the integrety of the metal springs. THey are usually rusty and sometimes the hog-ties that fasten then come loose over time. Sometimes good upholsterers will rebuild the spring bed during re-covering. The stock E9 seats are far from supportive in their best form, so it's well worth the extra effort to make sure the springs and filling material are upgraded.