Caffeic acid, C9H8O4 (CAS 331-39-5)is a naturally occurring phenolic compound, (formerly called carbolic acid). Carbolic acid was succesfully used as a photographic developer during the 19th century.

Caffeic acid is also a very effective antioxidant.

Coffee (specifically instant coffee) is an inexpensive source of Caffeic acid. Of course, instant coffee also contains caffine (useless as a developer) and various other contaminants.

There are many commercial chemical suppliers around that will be glad to sell you Caffeic acid from high purity research grade on down.

Green Developer

Water
Instant Coffee (Caffeic acid)
Vitamin C (Ascorbic acid)

You can drink it as-is:

or add Sodium Carbonate (washing soda) and develop film or prints with it. A Green for Gold version with higher purity Caffeic acid is also available at a very high cost.