Staples/Office Depot-type places are good sources. They aren't cheap, but they aren't totally ridiculous, either. I'm not sure what I paid for a pack of 10 anymore, but that kind of stuff also goes on sale occasionally too.
IF you care about archival safety (and I'm not saying you should, I'm just reporting the facts...)
Most office supply inkjet transp. are not archivally safe. 3M explained that they are made in an acid bath and that's what makes the slightly rough surface that the ink adhere's to.
So, if you care about the srchival safety and unless the box says "acid-free" (as Graffix does, you can assume they are not safe.
As a side note, you can always use write-on transparencies as these have no coating whatsoever. Check out the following links to see how to use transparency films -- even a film negative with your inkjet printer:
The first one I bought was a single sheet ($0.50) at the office copy store (CopyMax).
I have since bought some 12x12 Scotch 3M "printable transparent sheets for scrapbooking & Crafting" at Joann's on clearance. I haven't used them yet (and don't have a large format printer, so I'll have to cut them down to print ).
I just looked at the package. It says:
"Important Notice. Before using this product, you must evaluate and determine if it is suitable for your intended application. You assume all risks and liability associated with such use."
I would assume this means they are NOT archival safe.
It makes me wonder if the pre-printed transparencies sold for scrapbooking are archival safe. ???