Mark,

I believe I got this on sale at Harbor Freight Tools for around $10-$15. A very good tool for the money. You can switch between mm/inches and can be used to measure gaps as well as the widths of items.

They sell them on their website for $15.99, but you might see if there is a local store you can buy them at.

Here is the link: http://www.harborfreight.com/cpi/ctaf/displayitem.taf?Itemnumber=47257

Hope this helps!
Wilson