We need look no further than Saturn's rings.
Saturn's rings are made up of billions of particles ranging from grains of sand to mountain-size chunks.
Pioneer 11 passed through the outer rings and although it came close to hitting one of the two new moons it discovered, it survived, suggesting that the density of the outer rings is low enough to not give you the effect you're looking for. However, I believe the density of the inner rings are. The Cassini probe discovered evidence that there may be millions of moonlets in the rings. That suggests mountain-sized to me.
So, can we have a large field like in the Empire Strikes Back?
Of course we can. But there are some caveats.
Gravity is not your friend. When you get mountain-sized asteroids in close proximity, gravity wants to pull them together to form planets. This process takes a honking long time — but it's nevertheless going on. So why doesn't Saturn's rings coalesce? Because the gravity of Saturn and the orbit of the rings keep them churned up like a food mixer.
All that cool flying around in The Empire Strikes Back, without the use of Star-Trek-grade "inertial dampers" would turn most passengers on a space ship into a thin pink marmalade on the side of a bulkhead. This cool bit of space math points out that the orbital speeds of Saturn's rings are up to 24 Km/s. That doesn't sound like much until you convert it into Km/h (86,400 Km/h or Mach 70). This means you need a lot more space between those asteroids to give you maneuvering room than the folks at Lucasfilm led us to believe. So if you want "real," you're not actually looking for what the film did. You need the asteroids spread out much more (there'd still be plenty of high-stress flying around!).
I don't believe we know yet of any such fields outside planetary rings, but we're only just beginning to view our galaxy with detail. That being said, I can think of no reason why they can't. In fact, I think for the sake of a story it would be very plausible to find them in or around nebulae or orbiting newly-formed stars.