The rubber "o" ring can be found at your local plumbing supply, just bring the knob along and find a ring that fits. As for the plastic cover cracks, I recall seeing a web page of someone who reconditions these things...unfortunalely I did't bookmark it at the time and I can't remember who it was now. Hopefully someone else knows.
Yep, black from NPD for $55. Colors available for $60.
Of course they came out with these about a month after I FINALLY found a used one with "minimal" cracking for almost as much money. Only took me 3 three years of on and off hunting to find one. Figures. ::
I recondintioned one the following way. After a very complete cleaning to get all oils off, I used Zap-a-gap super glue, put a thin bead in the crack, and then sprinkled baking soda on top of it. This makes a very sturdy plastic like fill that hardens very hard after a bit. But prior to it hardening all the way, I sand lightly with 400 grit wet dry to bring it to the level of the surronding plastic. Once these are all filled and flush, I used my dremel with the small dental bur tip and carefull applied the grain in the areas that where filled. This is time consuming, but necessary. After, I masked off the chrome portions and sprayed the plastic with vinyle dye. Looked as new, and has held up a long time. Old modeling trick, works good for restoring broken portions of Lionel trains too.