Any idea what the wattage is ?
I'd check with Playthings of the Past:
http://www.oldradioparts.com/pg2a22.htm
They have several 40 ohm pots at 2, 3 and 5 watts. Also some 300 ohm at 2 watts.
There's also ebay.
Here's a 300 ohm 25 watt:
http://cgi.ebay.com/OHMITE-300-OHM-2...-/400092214166
It seems 40 ohm are a little harder to come by.
Here's a 40 ohm
http://cgi.ebay.com/300B-211-845-Nob...-/270495697121