Who publishes Usagi Yojimbo? In which format?
Usagi Yojimbo, written and drawn by Stan Sakai, is currently being published by Dark Horse Comics. It's published in black and white, and each issue has about 24 pages. 10 issues or so are released per year. Some stories are single-issue self-conclusive stories, some of them are 2, 3 or even more issues long, and some of them are full-length stories that take 8 or 9 issues. In the case of short stories, there's usually a bigger story arc that covers the stories and gives them a common ground.
How long has UY been around? How can I find older UY stories?
UY has been around for 21 years now. Most of the stories have been collected in graphic novel format, both in trade paperback and hardcover (although these books are pretty expensive and somewhat rare to find.) There are 2 publishers releasing the books: Fantagraphics books publishes the first 7 books, and Dark Horse publishes the rest. Here's the full list:
FANTAGRAPHICS:
Book 1: Ronin
Book 2: Samurai
Book 3: Way of the Wanderer
Book 4: The Dragon Bellow Conspiracy
Book 5: Lone Goat and Kid
Book 6: Circles
Book 7: Gen's Story
DARK HORSE:
Book 8: Shades of Death
Book 9: Daisho
Book 10: Brink of Life and Death
Book 11: Seasons
Book 12: Grasscutter
Book 13: Grey Shadows
Book 14: Demon Mask
Book 15: Grasscutter II: Journey to Atsuta Shrine
Book 16: The Shrouded Moon
Book 17: Duel at Kitanoji
Book 18: Travels with Jotaro
Book 19: Fathers and Sons
Book 20: Glimpses of Death
Book 21: The Mother of Mountains
Book 22: Tomoe's Story
Book 23: Bridge of Tears
Dark Horse also publishes Space Usagi, a futuristic UY spin-off, and "The Art of Usagi Yojimbo", a beautiful hardcover artbook [The hardcover is sold out from Dark Horse, but a softcover should be in the works. You can still try ordering it from our UY Marketplace dealers on the Buy UY page -- Todd]
There are a handful of stories that haven't been collected in books yet, most notably the UY Color Specials published by Fantagraphics. These stories take place at a point not reached yet by the main storyline, but should be reached around book 22 or so.
Sometimes some of the books are hard or even impossible to find. That's because they get sold, which is a good thing

Are there any differences between the comic-books and the graphic novels?
Not many. Comic-books are slightly bigger, which makes the art look better. Although I prefer the graphic novels myself. The most notable differences are in books from 8 to 10, which have stories first published in color by Mirage studios, and have been reprinted in black and white. There's at least one story (Demon Mask) which has had a few panels changed due to a research mistake.
How many publishers has UY gone through?
Four: Thoughts and Images (the stories of this period have been collected in Book 1), Fantagraphics, Mirage (reprinted in books from 8 to 10) and Dark Horse.
I want to start reading UY but I don't what to read first!
Well, the easiest way is to get one of the books and start reading! The books are sorted chronologically, so you may begin with book 1 and keep reading! However, most books are self-conclusive, so you may read any book you want first. If you'd like a good starting book instead of buying all the previous books first, here are a few suggestions:
-Book 2 (Samurai): how Usagi grew up and became first a samurai, then a ronin.
-Book 9 (Daisho): a bunch of short stories; one of them explains how samurai swords were made. Great entertaining and educational value.
-Book 12 (Grasscutter): the most awarded book of the bunch, the whole book is a single story about how Grasscutter, or the Blade of the Gods, was made, and how it falls in the hands of Usagi and causes a great political stir.
I wouldn't recommend reading any books beyond 12 first, because they might spoil some stories of the previous books. However, the only one which I absolutely don't recommend is book 15, if you haven't read book 12 first.
Cool! Now, where can I buy Usagi Yojimbo?
For comic-book issues, try your local comic-book store

-Fantagraphics Books: Publishers of the first seven books, they also sell online.
-Things From Another World: Dark Horse's online store.
-Heritage Source: for all things Japanese and Japan-related, including UY.
-Rabbit Valley Comic Shop: store specialized in furry and anthropomorphic comic-books.
-There's also the Dojoboard Forum! Go to the Marketplace and sell/buy anything UY related!
-There are many more online stores. If you know one that sells UY, post it in this thread and I'll add it here.
This should give you quite a bit of knowledge about the UY world

Other places to learn more about UY and the UYD:
The History of Usagi Yojimbo
History of the UY Dojo
UY Letters Column
UY Concordance