marex is a well known broker in london and they offer access to a number of OTC markets. They offer a number of OTC voice and OTC screen markets in energy. http://www.marexspectron.com/the-markets/energy
here in the states, look at Atlas (Axiom), down in Houston, or ET's own ogarbitrage who is with (I believe) Coquest in Dallas....a couple of names to start with in OTC energy stuff that are easy to find. FC Stone and Atlas both facilitate a lot of OTC trades in grains, a lot of that market is regional brokerages though and a of that biz is more physical focused (think JW Nutt Co). there are many more names. I used to be in physical grains and to a lesser extent, upstream oil. Edit: Progressive Fuels is another, focused on a lot of ethanol trading....big RINs focus