The USGS has an on-line program in one of it's ftp sites which can be used to calculate the
limits for a FE region.
Otherwise, the limits have been published in the following works:
E.A. Flinn and E.R. Engdahl: "A proposed Basis for Geographical and Seismic
Regionalization" Rev. Geophys. 3, 123-149 (1965)
and
E.A. Flinn, E.R. Engdahl and A.R. Hill "Seismic and Geographical Regionalization",
Bull. Seis. Soc. Am. 64, No 3,2, 771-993 (1974).

The first of these contains maps of FE regions, the second has the geographical
limits (numerical).
Last year when we were trying to produce shape files for GIS web applications of
Seismic data we did an extensive search on the web for the actual data. It does
not appear that there is (or at least was) any URL where the defining data is
located.