Welcome to OTT!
I looked into this a while back, and the most economical way to buy was through my local Kubota dealership. Of course, I was looking for spray cans, and they were cheapest, especially for Kubota paint. If you are looking for bulk, you should check with an auto paint jobber.
A regular old can of Rustoleum is $5-$6 around where I live, and I was buying Kubota paint at the dealer for about $9 a spray can. I didn't feel that was a bad price, considering.
The other issue, though, is with spray cans, you don't get hardener, so the paint isn't quite as tough as what you would spray through a gun.