Source for Variac

I have a collection of about 20 MacIntosh tube pieces. I rotate them in two systems to keep them running. However, I am told that I should regenerate the caps after storing for a while by gradually increasing the voltage with a Variac or equivalent. Putting full voltage on the caps can short them and the old ones are no longer available (are they?). Can y'all steer me to a source for a variac?