I just checked mine out and it is a 3/8 - 16 thread. These knobs will fit.
http://www.mrgasket.com/ProductDetails.aspx?brandId=1&productID=22511&majID=825&minID=&selection=23&minselection=
http://www.mrgasket.com/ProductDetails.aspx?brandId=1&productID=22461&majID=825&minID=&selection=23&minselection=
http://www.mrgasket.com/ProductDetails.aspx?brandId=1&productID=22457&majID=825&minID=&selection=23&minselection=
http://www.mrgasket.com/ProductDetails.aspx?brandId=1&productID=22466&majID=825&minID=&selection=23&minselection=
You can buy them at Jegs.
http://www.jegs.com/cgi-bin/ncommerce3/ProductDisplay?prrfnbr=1920&prmenbr=361
http://www.jegs.com/cgi-bin/ncommerce3/ProductDisplay?prrfnbr=1921&prmenbr=361
http://www.mrgasket.com/ProductDetails.aspx?brandId=1&productID=22511&majID=825&minID=&selection=23&minselection=
http://www.mrgasket.com/ProductDetails.aspx?brandId=1&productID=22461&majID=825&minID=&selection=23&minselection=
http://www.mrgasket.com/ProductDetails.aspx?brandId=1&productID=22457&majID=825&minID=&selection=23&minselection=
http://www.mrgasket.com/ProductDetails.aspx?brandId=1&productID=22466&majID=825&minID=&selection=23&minselection=
You can buy them at Jegs.
http://www.jegs.com/cgi-bin/ncommerce3/ProductDisplay?prrfnbr=1920&prmenbr=361
http://www.jegs.com/cgi-bin/ncommerce3/ProductDisplay?prrfnbr=1921&prmenbr=361