I bought 2 packs of 100 on eBay, so they can be found, but sounds like you need a lot.
I recently bought 1000' of polyester sleeving in a continuous roll for less than $40. They can be cut to length to faciliate your filing method but make sure you get the kind with the flap that opens on one side.
According to recent archiving wisdom, these are the preferred archiving method. The adhesive used in glassine/plastine (same thing I think?) can be detrimental over time. If you use these, make sure the adhesive edge is in contact with the base side and not the emulsion side.
The polyester are nice because they are completely clear, or you can get some with 1 frosted side I believe. They have no adhesive and are by all accounts completely stable for hundreds of years.