may want to check the library, and flip through some of their books... don't just do a search for slipcovers I'd say look through all but their garmet type sewing books look at the older books too, not just the new ones,
I actually have
Goodhousekeeping New Complete Book of Needlecraft by Vera P. Guild cop. 1971 ISBN 0-87851-002-8 and it has a section "sewing for the home" which goes into how to make slipcovers, tells you how to measure the couch/chair and sew them right up, all in 13 pages

would give you a good place to start though.
hope this helps, maybe someone else can help w/ finding an actual pattern