There's two 8608 flavors -- one is more eco-friendly, and isn't friendly to Brodak dope. One doesn't have the '-S' the other does. I can't remember which is the right stuff -- hopefully someone will chime in.
If you're close to someplace that sells finishing supplies for full sized aircraft, Brodak dope is re-branded Randolph dope. So the Randolph thinner should be just the ticket, and you can probably get it in 1-gallon or 5-gallon cans. If they don't normally carry dope you may be able to sweet-talk them into it, or you may be able to find out who the local distributor is.