Some good sources for fancy numbered notes:
Banknote Grading - A good site that tries to explain how to grade currency. We use this guide.
BEP Monthly Production Statistics - This is a great site for researching when different notes where printed
The Currency Gallery - A Great Reference with pictures and details of all U.S. Currency
Executive Currency - Great site for fancy numbered currency from "the grand master of special serial numbers".
First City Currency - Great web site for Fancy Numbered Currency and other info. Includes the BEP production numbers
Phillip Driver Currency - Great web site for $2 Legal Tenders, Large-sized notes and some Fancy numbered notes.