Variable sets

Dear all, in the paper "Variable set theory" from Barr/McLarty/Wells we find a sketchy description of an example "the set of businesses". I wonder if this example is described somewhere in more detail and if there are other examples of "common usage" somewhere in the literature that may be even approachable by non-mathematicians? Can somebody supply a copy of "Variable Quantities and Variable Structures in Topoi" from F. WILLIAM LAWVERE? Best regards Uwe Wolter [For admin and other information see: http://www.mta.ca/~cat-dist/ ]