Tom, I found this site (
http://www.electro-nc.com/products/encoders.shtml) Might be of some use but I don't have the time to really dig any more on this.
That being said, the lack of full documentation on the linked site worries me. Most reputable sites provide a little more information on their products. Also, quadrature encoders are usually MUCH more expensive which sets off another flag for me. I would be careful ordering from this site, if you do make sure you monitor your credit card well after that.