There are lots of sites if you Google it. The ones worth money are hard to find. You can look at eBay's completed items for ladders and repeaters and such. Good luck. I haven't had any. I seem to think people buy the new money and pull out any good bills. If you have access to buying fresh bundles you might do excellent!!!
Other than collectible serial numbers are Error Notes that include Gutters (or Creases), Offset Transfers, Ink Smears, Shifts or Misaligned Overprints, Mismatched serial numbers, Inverted serial numbers, Inverted face, Printed folds, and several more different types of error bill that are highly collectible.
Don....