I have a garmin etrex legend hcx. Similar to what you have, but I think your device does not have a micro SD slot, so any additional maps need to be loaded onto the build-in memory, which according to the spec sheet is only 24MB -- not much.
As I think you know, the official Garmin product is City Navigator North America. When you buy this and use the PC software for loading it onto your GPS, you can be selective about what you load. But it's really expensive.
I looked up some alternatives, and this looks really promising:
Free routable maps for Garmin brand GPS devices
On that site, you navigate a world map and then select area tiles to be assembled into a link that you download. The data is based on Open Street Map data. I haven't used OSM data myself, but from what I've read that its quality varies -- sometimes it's supposed to be even better than commercial products, sometimes worse. I don't know what the western PA data is like, but it seems worth trying. If you do, write back to let us know how it goes.