About 3 years ago I found a roll of flat plastic tube on Ebay. It was listed as "120 roll film sleeving" I'm not sure what its original use was. Maybe it was from a defunct commercial lab. It seems to be a very similar type of material as the Print File brand. The roll has got to be at least 200 ft, and it does a great job protecting my negs. Icost me less than $20 too. I guess I got lucky.
Printfile actually sells this still. How do you store them after that though? Rolled? In some type of case?