The Allegheny County Marriage License bureau has an index to marriages and can also provide a copy of the marriage license. You can search their index yourself for free, or they will do it for the cost of the license copy (which was $4 last time I checked). If you're planning to get a copy of the license anyway, it makes sense to have them do the search, since you have both names and the approximate date, and the search is included in the cost of the license copy.

And it would certainly be cheaper and easier than using the FHL microfilm.

Their contact info:
Allegheny County Register of Wills (Marriage License Bureau)
City-County Bldg., 1st Flr.
414 Grant St.
Pittsburgh, PA 15219
412-350-4232
www.county.allegheny.pa.us/regwills/cert.asp