I don't think that is anything you will find New or even NOS. This guy built his own:
http://www.99main.com/~allens/gridleak.htm
Have you tried running the radio with the 7 meg resistor in? I don't think they change much in value, being carbon-coated glass, so it should run as well as it did originally. You could also parallel in different resistors until you get 3 meg.