From the site:
Currency notes of denominations above $100 are not available from the Department of the Treasury, the Federal Reserve System, or the Bureau of Engraving and Printing. On July 14, 1969, the Department of the Treasury and the Federal Reserve System announced that currency notes in denominations of $500, $1,000, $5,000, and $10,000 would be discontinued immediately due to lack of use. Although they were issued until 1969, they were last printed in 1945.
These notes are legal tender and may be found in circulation today; however, most notes still in circulation are probably in the hands of private numismatic dealers and collectors. If you are interested in purchasing these larger denominations, it is suggested that you contact private dealers or collectors who are usually listed in the classified section of the telephone directory under the headings of “Coins” and “Hobbies.”
(Via Boing Boing)