This article is from the Paper Money Collecting FAQ, by Bruce Giese email@example.com with numerous contributions by others.
See the section on societies and the section on how to get started.
The main periodicals are Bank Note Reporter and the IBNS Journal.
The SPMC and LANSA have periodicals, too.
For Green Sheets (see Section 1.15), you can contact:
The Currency Dealer Newsletter
P.O. Box 7939
Torrance, Ca. 90504
1 yr subscription = $44
2 yr subscription = $78