Lots of the cast iron dutch ovens get listed on evil-bay all the time - both new ones and old ones. You can get a pretty good deal if you watch the listings a bit. A couple older company names to look for are Griswold and Wagner. They were two of the major manufacturers here in the States during the early 1900's.
Over on this side of the pond, the current new brand out there is by a company called Lodge - with their name cast in as part of the lid. Lots of the camping supply places carry them. And the Cabela's stores have also been getting some cast iron pots, kettles, and dutch ovens from South Africa that are marked Potji.
Now, I and a couple friends are looking to find the very early French version of a "bake kettle" - which often gets blended in with and confused with the "dutch oven". It is called a Tourtier. It often has a copper or sheet iron bottom pan with iron feet riveted on, and the cover is also made of copper or sheet iron, extra tall, and with the top formed so that you can pile coals on top for baking - just like those cast iron dutch ovens. That lid often looks like the cover you put over a cake on a serving tray to preserve it.
These were used back in the 1700's, 1600's, and earlier - before cast iron became more common. You would rake some coals out to the side of your fireplace, set the tourtier (bake oven) over it, put the food to "bake" inside, cover it with the lid, and then pile extra coals on top so that it heats from above and below - just like you do with a modern cast iron Dutch Oven. My friends and I are looking for these to use at our Historical demonstrations for the 1600-1700's French Colonial time periods in the areas that France controlled across North America, especially along the Mississippi River where it flows through/along Illinois and Missouri - Kaskaskia, Fort de Chartres, St. Louis.
Our search has not been too sucessful. The few we have found are on display in museums. Right now I'm thinking through what I will have to do to make several up myself out of sheet iron.