Look in your phone book for either architectural salvage or scrap metal yards. You can often find old radiators there.

Just FYI, on a steam heat system you don't need to use steam radiators, you can also use radiators designed for a hot water system. You'll just need to plug the bleeder valve hole and the outlet and drill for a steam vent.

This person is interested in downloading theirs. Check out the link below. Also look on Craig's list. You may have to pay a few $, but most people are so happy if you cart the behemoths away that they give them away almost for free. That's how I got my beauty ( for a hot water system).

http://brownstoner.com/forum/archives/2007/01/steam_radiators.html#comments

How close are you to White River Junction,Vt? If its not to far here's a place you might find interesting.

Here is a link that might be useful: Vermont Salvage

Try New England Demolition and Salvage. It's near the beginning of Cape Cod. I bought 10 radiators there when I extended our steam system and needed to rebalance some exisiting radiators. The guy who owns it takes all the radiators off working systems himself - hence, no broken/cracked ones and they are all stored inside.