My screenname is all about how I accumulated equipment to farm with, as well as how I suplimented my income while raising 3 kids. I bought used equipment, some of it in less than mint condition, repaired it, kept the best pieces, and sold the rest. Over a 30 year span, I probably bought and sold 1000 implements and 100 or more tractors. I managed to accumulate some really nice equipment for my own use and made a few bucks along the way.
I never found one single source that stood out as the best.
Auctions, if you know what you're doing, can be a place to start. Understand most consignment sales are an avenue for people to unload junk. Estate sales are a better source for usable, intact equipment, but in many cases, the stuff has sat for a few years. Also, in the case of estate sales, equipment may have been in a state of deterioration as the owners health declined.
Newspaper adds will bring a bargain or two. You have too take what they give you. Don't expect a bargain every edition. Obscure, misworded adds always netted me the best buys. (People either didn't know what they had to sell, or other potential buyers didn't recognize what was selling)
"Equipment trader" type publications are common. Find the ones in your area and keep an eye out for bargains.
Go have a "heart to heart" with a few dealers in your area. Explain what you're looking for. Leave a name and number with a salesman. They get people wanting to trade in equipment all the time. Sometimes they want more than a dealer can invest in a used piece, but less than what you as an individual will be willing to pay. If the dealer is a horse trader, he'll put you on to the item so he can make his sale too.