How old are you wanting?
Check for Craigslist, Facebook, all the normal places.
Also hunt for local Farm Auctions.
Everything is a lot simpler with the older tractors than newer ones. However, there will be few computer hookups to tell you what is wrong with them, so you'll be on your own to figure out what is wrong.
However, also consider a lot of parts can be worn which could lead to time consuming and expensive repairs.
I'm working on a John Deere 440ic. Probably from the 1950's. I got it for about $1500. It was supposed to have low hours, but something is definitely wrong with that. It ran, but very poorly. It is barfing a lot of oil up the exhaust. So, I'm expecting a full engine rebuild. I found a 420c engine at a local parts dealer for $1500 that should fit it. It turned over, but once I got it open, I could see excessive cylinder wear.
OK, I've got 1 or 2 engines to rebuild.
One of the steering clutches wasn't working when I bought it. I've now adjusted it to 100% adjustment which was barely enough to catch. But, that will mean a new clutch pack, or two or three.
All the bucket bushings are very worn. Probably OK for my use, but I'd like to fix them if I'm going to ever sell it. The tracks seem OK, but I'm not an expert.
Anyway, I could well have $5000 invested in my $1500 dozer before it is really usable.
Now, on the good side... there are mighty few new dozers available in the USA under 10,000 lbs, with that class largely replaced by tracked skid steers. Buying an equivalent dozer less than 20 years old will put one between $20,000 and $100,000.
You may be best if you find a tractor that somebody has already rebuilt.
A person can put a lot of time and energy into rebuilding a tractor. Then when the widow tries to get rid of them, they're just not worth hardly anything. Yep, one of Mom's neighbors just did that.
The other thing to consider with the older tractors is how you'll be using them
Back until the 1970's or so, everything was 2 wheel drive, with split brakes to transfer drive from one rear wheel to the other.
Kubota started bringing in 4x4 tractors, then everything went to 4x4.
Lots of really good, quite powerful old tractors, but all 2WD.
NO ROPS.
The trike tractors may be fine for row cropping, but keep away from them if you're on a hill.
There should be some small utility tractors around that haven't been driven into the ground. Perhaps some yard work, or tilling a small garden. However, if less than 20 or 30 years old, they'll still be expensive.