There are lots of great nurseries and garden centers in the area. I'm very fond of Gettysburg Gardens. Also try Alloway Creek Gardens and Willow Pond http://www.willowpondherbs.com/ .

There are several others!

Here is a link that might be useful: Allowaycreek

Just west of G-burg on Rt. 30, almost to Chambersburg is Snavely's. Look for it on the left going west, not too far after you go down the west side of the hills into the flat ground. Well worth the trip for flowering trees and shrubs. I also go to Boyer's (on the web) near Arndtsville, about 10 mi. N of G-burg. Nice selection of trees and perennials, and decent prices. A little farther up 15, just past Dillsburg there is a good one for perennials. Ashcombe Nursery, on Grantham Road.

All the businesses mentioned already are good, though I don't have personal experience with Snavely's. Last season, I discovered Seasons Past Farm and Gardens, which is near Alloway and Gettysburg Gardens.
www.seasonspastfarm.com
They have a very nice selection and are very helpful -- lots of hostas and trees.

Both Snavelys and Boyers are very good. Boyers has lots of unusual trees and shrubs. Snavely's is better for perennials.

If I'd remembered their names, I would have mentioned them sooner!

Trayer farms is south of Mercersburg PA (maybe 20-25 miles from Gettysburg?) and the best one I know and we have hit them all. They have at least 8 big greenhouses and many, many outside perennials arranged alphabetically by their botanical name. They don't have just one variety of each type either. I am guessing they have 10-15 varieties of sedum alone. If you check the greenhouse some have seed started perennials at reduced prices. Limited shrubs and no trees (that I noticed). They are open Sunday afternoons too. You could hit Snavelys and then go 997 south to Waynesboro, stop at Twin Kiss to get ice cream and see the deer, go right on Main street head toward Greencastle then toward Mercersburg. Trayers is on 11452 Welsh Run Road (use mapquest). It's a nice ride. There's a new greenhouse opening up between Zullinger and Greencastle this year too - plain folk so no Sunday sales.

Thanks for all of the wonderful recomendations ! I will most likely try them all...