If you want foreign bills that are 100 years old or more for under $10, I highly highly recommend Imperial Russian banknotes. The 1898-1915 period saw such a ridiculous rise in inflation that many of the 1, 3, 5, 10, 25, 100, and 500 Rouble notes can be had for very little (compared to other notes, and as I mentioned in another post the prices on these are starting to increase). These notes are absolutely beautiful, too, considered cutting edge printing in its time.