For High-end pure Oud oil, Agar Aura and Ensar are generally considered the top in terms of quality. These two companies can have quite different ways of doing business, and so some people prefer one over the other.
For more affordable pure Oud oils, again Agar Aura (some are very high quality at reasonable prices), Oudimentary (medium quality at very good prices), and a recent newcomer Feeloud. There are some larger producer/sellers like Asiaforestry and Oudline that can be well worth a look. There are a lot of dodgy operators out there too - so beware. There are also some who trade on the mystique and rarity of Pure oil to charge really exorbitant prices and even start with an average oil at an average price and then raise the price as the oil becomes in shorter supply.
It's probably a good idea to read up on the various types of Oud oil (including but not limited to: Borneo, Cambodian, Thai, Hindi, as well as wild vs cultivated), and some of the above vendors also offer samples - which is advisable given the prices some oils go for. There are currently some exceptional oils available for under $400 (even as low as $165).