[Electronic M]

Some of the Sola constant voltage isolation transformers are good when the line voltage fluctuates.

Isolation trans show up at antique radio and ham swapmeets regularly usually for cheep.

In a pinch, if you have two step down or step up transformers with the same secondary voltage (identical transformers are best) you can connect a secondary from one to a secondary of the other, plug the primary of one into the wall and the device you want isolated into the floating secondary... filament trans are nice for this because you can hook a low voltage lamp to the secondaries and use it as a power indicator...Can even steal heater voltage for experiments off of the low voltage windings. I hear a pair of microwave oven trans will make for a very ballsy isolation transformer (something that can handle ~500W+ watts easily), but the ~3KV AC on the secondaries, when powered, is lethal to touch so building an enclosure is IMO necessary for safety.
I've also used this hack to power up transformer radios with open primary/line voltage windings...All that you have to do is connect a heater trans of the right voltage to the radios heater line and the B+ winding if still good will turn that into B+.

BTW even a good isolation transformer can stop isolating...I had one that appeared to be made in the 30's-50's that I used for a number of years until one day a chassis I was servicing bit me...I took out my DMM and discovered there was significant Primary to secondary voltage. It really surprised me that it could fail like that, and goes to show you gotta test your test equipment for failure sometimes. It is part of the reason I like that hacking two transformers into an isolation trans trick...In that method there are two points of isolation doubling the breakdown voltage, making it twice as hard for primary to secondary leakage, and providing a failsafe isolation if one of the two trans develops a primary to secondary short.