Might want to splurge on a new rheostat... eBay is your friend, and Google is very helpful also...

Not sure if you could use a 120V AC dimmer; might have too much resistance.
How about:

Those cheap halogen torchiere lamps that people always throw out when they move (there's one on the sidewalk on every block in any major city at any given time)?
  I think those are low-voltage and the dimmer circuit is post-transformer... usually all that's wrong with discarded floor lamps is that the bulb is burnt out or the super-chintzy structure has failed because the cat touched it.