The ECF DIY guide lists a bunch of suppliers. I recommend you read the guide before buying flavors. Some flavorings are unsuitable for vaping (some contain a butter/custard flavoring component diacetyl which is only safe for food and cosmetic applications, and some are not water soluble and are hard to work with)
The descriptions at Perfumer's Apprentice also note which flavors are not suitable for vaping or not water-soluble but some other suppliers of flavoring brands do not.
DIY e-liquid