ABraThatFits at Reddit – The leading anglophone forum for advancing fitting theory and practice. Check the sidebar for comprehensive sizing and shape resources. Nose around for general education.
Breakout Bras Fitting Guide – A good first step. Excellent one-page resource on how to identify fit problems, size yourself correctly, and put on a bra properly.
Butterfly Collection Bra School – Good selection of resources on fit offered by a Canada full-bust boutique.
Bra Naked Truth at A Sophisticated Pair – Good selection of resources on fit offered by a North Carolina full-bust boutique.
The Bra Band Project – If you just can’t, CANNOT, believe the size that’s being suggested for you by a reliable resource, look here.
Busty Resources Wiki – Does what it says on the tin.