Don't try to save a few bucks buying the generic equivalent. Get the transparency film recommended by your printer's manufacturer. This is especially important for ink-jet printers. The surface of the thin, clear film needs the right pores for ink absorption. Buy the right printer, if possible.
Some printers produce better overheads than others.
If your office gives presentations often, consider buying a printer just for presentation materials. Tektronix Inc. of Wilsonville, Ore., makes my personal favorites, the Phaser 350 and 360, which melt solid-wax ink blocks onto media in a thin film. Coverage is even and translucent. Avoid complicated backgrounds. Backgrounds should not call attention to themselves.