The counter knob is a 1K potentiometer with a knob built into it. Here's a link to the spec sheet. 
http://www.bourns.com/pdfs/3610.pdf.
You can buy them at Mouser Electronics. Mouser part # 652-3610S-1-102. The only problem is that they sell for $60. I found a few on some broken lab equipment that was being thrown out.