Flageolet beans are small immature kidney beans with a history that dates back to the 1800’s in France. The were very hard to find in the US but have become more common in the the last few years. You can find the beans fresh, dried, or canned. The bean is available in a variety of colors including white (flagolet blanc), black (noir), yellow (jaune), red (rouge) or green (vert). Flageolet beans are traditionally served with lamb. More botanical information here.
Look for the dried beans at Whole Foods Markets. Walmart stocks them in some stores as well as online. There are several sellers that offer the dried beans, canned or jarred on Amazon.com: Flageolet beans
Drain the soaked beans then cover them with at least two inches of fresh cool water. You can add seasonings at this point or simply bring them to a simmer and cook them for about 30 minutes more or until tender.
If you need a substitute for flageolet beans your best options include: