If you want to trip to Ubon Ratchathani from Pattaya by bus, you can use bus operators which have route to Korat from Pattaya are as folloiwngs;
Srimonkol (SMK) : The bus terminal is located at Third Road, just close to intersection of Third Road and Pattaya Klang (Central) Road. The bus start from Pattaya, not from Rayong of other many bus operators.
>> Go to Srimonkol (SMK)
Nakhonchai Air : The bus terminal is located at Sukhumvit Road, close to three way intersection of Sukhumvit Road and Pattaya Klang (Central) Road.
>> Nakhonchai Air