Vellum isn't used with inkjet printers(or not recommended at least).
Vellum is meant for laserjet prints as far as I know.
There are tons of places online that sell good waterproof film meant for printing film positives.
you can try:
Silk Screening Supplies .com, Screen Printing Equipment, Silk Screen Printing Kits , (Ryonet), Try their waterproof-R Film I think it is.
Film and Supplies | Screen Printing | Victory Factory , their film isn't as thick as Ryonet's, but I have been using their films for a few months and I dont' have a problem with them.
There are a ton of other places, but I can't remember them off the top of my head.
Just remember to look for "waterproof" output film. non waterproof film might make a real mess of your printer and they take forever to dry. Waterproof films seem to dry before they are even out of your printer.