Is there a definitive site you all use for information on canning tomato
sauce?

Usually, I just grind the tomatoes and make sauce and freeze it in gallon
bags. I want to try canning the juice/sauce rather than adding all the
ingredients for sauce, cooking it down and freezing it.

I would think that you must cook down the juice for a bit and then can.

One other question-can one spice the juice, say with onion and garlic,
before canning or is there a real problem with botulism with garlic?

I usually do around 2 bushels-want to expand to 4 or 5. I do have an
industrial size electric grinder (50 bushels in 8 hours-or so say the
literature it came with).