National Notary Association
The National Notary Association is the largest and most recognized Notary related association in the country.
It is my go-to source for renewing my commission every four years, to buying my stamps and insurance through the NNA, etc.
Notary Stars is the newest kid on the block and is doing a great job of offering notaries the kind of training that will make them better signing agents.
In addition to all the free resources, they also offer a membership program that dives deep into notary document training.