Is the idler wheel missing or do you need one to replace a bad one? If you have one, you can sometimes sand down the rubber to get rid of dents, and soaking in brake fluid sometimes does miracles. But the long term solution is to have it rerubbered - there are several people out there who do this - years ago I used Terry's Rubber Rollers.
If the thing is missing, there are various sources for replacements, Gary at Voice of Music being a popular choice here. But just googling will probably pop up other sellers.