RF modulator are on ebay:
http://search.ebay.com/rf-modulator_W0QQfkrZ1QQfromZR8
Good experience I had with the Jensen modulator. U.S. modulators have Channel 3 or 4 output, European have UHF output. But it is much better to choose a VHF output, color is not drifting away with this.
I don't know the powerline voltage, perhaps a transformer for 115 VAC is necessary (minimum 500 Watts for old tube tv's, for a Porta-color 200 Watts are enough). And 75 Ohms to 300 Ohms transformers are neccesary too for old tv's. These are the "basics" for operating U.S. tv sets.