It also depends a little on what you are wanting killed. If just mainly grasses, Cleth is cheap and effective. I've used Imox and its ok but probably wouldn't spend money on it again. Many weeds can just be controlled with mowing. And many of those weeds are deer friendly. But then I don't worry much of weeds or grasses in my plots. Deer told me they didn't care.
But to answer your question, I buy from Keystone and most other chemicals other than Imox from Rural King. Convenient.
Amounts are 6 oz Imox with surfactant and AMS gave me a great kill. I don't find AMS necessary but I like adding the sulphur to the crop. Good luck.