Over here the only state that might not have much in the way of solvent based paints and varnishes would be California. Otherwise any paint store, Home Depot, Lowes, or good hardware store would have it.
I agree that the oil based varnishes are still more durable than water based. Although water based paints have come a long way in recent years. In the early '90s I used Pratt & Lambert oil based white on a job we did in Philly, because the client specified it. It lasted only a few years before it had to be repainted. We followed the guidelines of the manufacturer to the tee as the project was an historical restoration. The paint yellowed rather quickly, maybe a year or two.