From my experience it has always been cheaper to buy a 5 or 10 amp variac fom a surplus place and enclose it in a box with fuses and outlets. Finished units from somebody like Staco Energy are often over $300, a surplus variac can often be had for $75 or less.
For an isolation transformer it is fairly cheap to buy what is known as a control class transformer, wired to provide 240 / 480 to 120 / 240 , and use the split windings to hook it up as a 120 to 120 isolation transformer. These transformers come with 4 terminals on each side with split windings, so the output volage/VA rating of the transformer can be chosen.