BPS or Van Sickle are the usuals around here. Between TSC and all the rest of the Ag supply business's here carry one of the 2. Either will work satisfactory, and at a relatively low buck.
IF...you have all the right equipment to do your painting, and you're painting an entire tractor or implement, there are much better paints, with a MUCH bigger price tag. Once again, IF...you clean, sand, prime, sand, prime some more, sand some more, and so on, these expensive paints will last longer in most cases, keep their shine longer, and protect longer. It's a big jump from $25 enamel to $200 base coat/clear coat in terms of what you need to know and have to spray it.
For "the average guy", the stuff from TSC does the trick.
(After painting a few things, I'm learning the uses of different types of reducer. Mostly to match with climate conditions. It helps if you can find a good automotive paint dealer with a helpful counter man. Bottom line is, you can't go too far wrong with the thinners recommended by paint manufacturers)
BE CAREFULL IF YOU USE HARDENERS IN YOUR PAINT! (read directions CAREFULLY)