Today there is a growing trend for many of them to make and bottle Champagne under their own label from at least some of their grapes. These are called "Grower Champagnes" and there are some great wines and great deals available.
Much Champagne is, not surprisingly, drunk in France, just like lots of Maine lobsters are eaten in Maine and nearby (and incidentally, lobster and Champagne is a great match!). In France, most of the Champagne drunk does not come from big well known Champagne houses but from a sea of Grower Champagne. More and more Grower Champagnes as well as smaller cooperative Champagnes are finding their way out of France, to the US, the Far East, and other places too.
Grower Champagnes are not universally great by any means. I've drank plenty in France that were fair to middling; not bad but not worth exporting. Kind of like the tomatoes I grow in my back yard are pretty good but not worth shipping any distance. With about 5000 different Grower Champagnes it should be no surprise that most of them are not that exciting, but there are some real finds among them.
How do you spot a Grower Champagne? Check the label for the words Récoltant-Manipulant (or the initials "RM") or "Proprietaire-Récoltant." These translate roughly to "bottled by the grower." Some growers buy a small percentage of the grapes they use, for example Diebolt-Vallois, and Guy Charlemagne. They are not allowed to use these special designations, but most people consider them close enough as do to be Grower Champagnes.
And how do you know if a Grower Champagne is any good? The easiest way is to taste it - drink some. If you like it, it's good. You can also go from recommendations by friends, wine store employees, and professional wine critics, but the bottom line is whether you like it or not. If you like it - it's good. All wine is that simple.
Large Champagne houses, by necessity, blend grapes from different regions to produce a Champagne that is always the same. For example, Moet & Chandon White Star always tastes the same, year after year, and it better - that is what consumers expect. It's a non vintage (NV) Champagne as well, so grapes from different regions within Champagne as well as different vintages are blended to produce the sea of White Star out there. Some critics claim that big houses remove or numb the character out of all but their most expensive wines in order to create identical wines year after year, whereas Grower Champagnes offer much more character and personality. There is certainly some truth to this, although many big houses can and do also put out quite tasty entry level Champagnes as well (and for the record, I like White Star!).
Depending on where you are will determine which Grower Champagnes are easily available, and you may be able to order them over the Internet as well. Some of the better known ones which are also very good and fairly widely available include Diebolt-Vallois, Pierre Gimonnet, Larmandier-Bernier, Egly-Ouriet, J Lassalle, and Guy Charlemagne. There are well over 100 available in the US today for example, and many in most countries that have an active Champagne drinking population.