Hi Ohiotuber,
I found vintage new stock bait casting line on Ebay. They appear to be one spool attached to the next, like backing, 50 yard spools. One I saw was 600 yards of 30 lbs test for about $27, buy it now free shipping. I also saw 9 spools of three different weights, with the current bid of about $5, but shipping was extra.
I also found some by just typing in casting line.
With either of the above options you should be able to get lots of backing fairly cheap. If it sells before you get there, I wuold guess it will come up again. One guy wanted $225 (or best offer) for two spools of vintage line....good luck to him on that....a little steep for backing by my tastes.
For what it's worth, there was also 3/16" anchor line. One spool of it would be backing for a lot of fly reels. The problem I saw was if you ever got into the backing, it might be interesting to see what happens when it hits the guides. (Just kidding.)
Regards,
Gandolf