My DH got me a pair at his favorite metalworking tool supply store. Or as Sienne suggested, look at this page.
If you are using the BD syringes, there is a little ring around the top of the syringe that can get in the way of measuring, so you use the calipers a little differently. Instead of using the bigger arms that measure around the outside of something (as shown in the post on using calipers), you use the pair that measure from the insides. As a result, you want ones where the inside arms have finer points.