The most reliable dance instructions will always be found in published books. Ball booklets are useful, but can be misleading or incomplete. Be aware that most historic dances are reconstructions, and there may be significant differences between versions. Here are some online sites that contain dance instructions:
Alan Winston is a caller and choreographer in the California Bay Area who has an extensive online collection of his dance instructions, including English and Contra dances.
THE ROUND is the group affiliated with the University of Cambridge in England. They have instructions for dances in their core repertoire, plus a page of caller resources. New callers may want to check out the notes on calling.
Choreographers' online dance instructions:
Choreographers without online instructions:
If a picture is worth a thousand words, then a video must be worth ten thousand words. While videos are a very fast way to learn dances, they may contain the wrong figures or wrong phrasing and other errors. Always cross reference with a published source whenever possible.
Lambertville catalogue of dance videos is an alphabetical and searchable list of dances. In addition to linking videos on YouTube and elsewhere it also contains the essential dance information (formation, meter, and links to instructions if they are available online).
Up A Double contains computer animations and verbal descriptions of many dances, and is a good place to compare different reconstructions of a historic dance.
YouTube has many dances, enter the dance name and 'English Country Dance' in the search bar.
The Childgrove Dance Video archive has about 200 ECD videos.
Online dance lists containing everything but the instructions can be useful if you are researching a dance.
The CDSS now sponsors Anthony Heywood's database of over 20,000 dances. For each dance it includes the formation (e.g. duple minor), a list of figures used (w/o instructions), published instruction sources, and a list of audio recordings of the music.
Scott Higgs and Jenny Beer have a listing of their core dances showing the formation type, musical meter and key, and source of sheet music.
There are several sources for buying dance books and recordings.
The CDSS store should be your first stop, it has both books and recordings, and CDSS members enjoy a 10% discount.
Bandcamp allows musicians to sell their recordings directly and lets them keep more of their earnings. Some recordings can be steamed live, and you may have a choice of just paying for a download or getting a CD in the mail. You can buy a single track instead of a whole album. The English Country Dance collection by Bare Necessities has many dances from the core repertoire.
Apple Music offers streaming with a subscription, or sales of tracks and albums in the iTunes store (on PC, Mac, or phones).
Spotify also has some English Country Dance music for streaming, you will need either a free subscription (with ads) or a paid subscription.