Hobby Express in Cranberry is a must - they recently just expanded the store and have a large assortment of O scale, plus all things hobby related. Its just off the PA turnpike exit 28 and near I-79.
And while in this direction, C.T. McCormick, an old-school train shop section inside and old-school hardware store, is only a 15-20 minute drive from Hobby Express.
And if in this direction and in the mood for some 1:1 train activity, you could continue on to the Greenfield Railroad Museum and Park. The park can be visited pretty much any time -- the hours listed are for the indoor museum and gift shop.
Also in this direction, but a little out of the 90 minute drive window (approximately 2 hours), is the Corner Field Model Railroad Museum & Trading Post Train Shop. The shop is small, but the layout is huge and well worth a visit. Be sure to check hours and if open as they sometimes have to close due to family issues or obligations.
If planned and timed properly, all 4 of these places can be visited in a single day trip (start at CF at opening, then Greenfield, CT and then HE).
Andy