Have you tried looking at the Campbell-Hausfeld web site (chpower.com)? I think they sell pressure switches there for some of their units. If you don't have a manual to use to identify the part you need, they also have some manuals available for download.

If you knew just exactly what you needed, then you might get one conveniently from a regular supplier. Grainger, for example, shows oodles of different pressure switches (specifically for use with air compressors) in their catalog.