NER Live Results (see live results from Rallysprints, Rallycross, and other events)
FrogRacing - Emmanuel Cecchet is an awesome resource for information, videos, reviews, and guides on how to make your build meet ARA or CRC racing rules.
Rally.Build - Grant is a great resource for racing gear, safety equipment, etc. Rally.Build stays up to date on the ARA racing rules and bulletins and carries most things needed to meet ARA technical and safety rules.
Demon Tweeks - one stop shop for racing gear, safety equipment, etc. Note that this company is located in the UK so return shipping can be expensive
Primitive Racing - an excellent resource for skid plates, differential protection, etc.
There are a few sites and resources out there for buying used rally cars, the most helpful as of recently is probably the North American Rally FS/FT/WTB group on Facebook. From time to time, you will find fully build and ready to race rally cars there for sale.
Special Stage.com - this is an older forum / resource that still has some activity, but it seems most of the cars for sale end up hitting the above Facebook group.