They're filled straight from the tap, sealed with a cap, and the ones in Morgantown cost around $15-$20. That includes buying the actual container, and when you bring it back to get it refilled - depending on the type of beer you want - it's generally cheaper. They'll keep in the fridge for about 7-10 days or 2-3 once opened.
Right now, in Morgantown, there are four* places to purchase growlers:
Morgantown Brewing Company is the only brewpub in town right now that actually brews their beer here, so it's perfect for a growler. My favorite is their Kettle Bottom Brown.
The third place that sells growlers is Black Bear. They don't brew their own beer, but they have a lot of local beers on tap, so it's a pretty cool option if you want, say, Chestnut Brew Works. Hopefully that's actually not the case for much longer, as Chestnut will be opening a taproom in South Park soon.
**Since this post, Chestnut Brew Works has opened their taproom, and they DO offer growlers. You know I got one.
Any others in town?