Anchor Brewing (of Anchor Steam fame) makes the best English Porter I've ever had. It's tough to find, but it's well worth whatever it costs if you do.
Samuel Smith's Nut Brown Ale is pretty good...though dark(er than Miller Lite), it's not at all similar to an Imperial Stout. In the same vein, check out Brooklyn Brown Ale. It's like Newcastle, but much better (nuttier, slighly sweeter, and with a better balance of malt and hops). My wife does backflips for it.
Lancaster (PA) Brewery's Milk Stout is damn tasty, and should be available in VA. It's a "sweet" stout (as opposed to Guiness, which is a "dry" stout) that's brewed with lactose, so it's not for the lactose intolerant. Dogfishhead (MD) brews an "Indian Brown Ale", which is basically a Coke-colored beer made like an American IPA (high alcohol, and hoppy as hell). The taste is pretty good and I consider it worth trying.
Beyond that, you're pretty much on your own. Tons of micros put out darker brews, but availability in any given area is a crapshoot. Certain lighter beers have gained a decent national following the past few year (like Fat Tire and Dead Guy Ale), but the dark ones are still and aquired taste. If a micro that you like puts out a porter or a stout, then give it a shot. You'll quickly figure out which ones you like. Most dark beers are pretty good, if you like dark beers. The tastes of those styles are more complex, which makes them much more forgiving of brewing mistakes. Even a bad one tastes pretty good.
Most of the imports that can be found are commercialized beers like Guinness or Heineken Dark. They're macro-brews produced for a wide audience, and generally not worth your time. The exceptions are the Vienna-style dark lagers. They're similar in color to a Newcastle, but being lagers they're a little crisper and cleaner in taste. Some of the best come from Mexico (a throw-back to Maximilian I)...Negra Modelo is a decent commercial example that's widely available.
Find a bar that has an ecclectic set of taps (or a six-pack shop that specializes in micros) and start sampling. You like dark beers, so pretty much anything you try will at least be OK (certainly far better than Coors light). Pretty much anything that isn't a "dark" American macro-brew (i.e. Michelob Dark) should be pretty good, and you'll quickly figure out which ones you like.