In Japanese it is usually called a Keikogi (稽古着) or Dogi (道着), but most people just say "gi".
We use a Judo gi, either single or double weave; but you can use a Karate gi; they don't stand up to the grabbing as well. Gis usually come pre-bleached but you can buy an unbleached one, which is cheaper, but which looks a bit yellow. They are sold with a white belt included.
There are any number of sources on the Internet. The price range runs from $30 to $500. Go for the cheaper ones. Most men are a size 5; most women are a size 4; but check with the size chart on the website, or just try on someone's uniform for fit.
Putting on the gi:
Pants: When putting on the pants, the kneepads go up front, as do the loops that the tying cords go through. Cords go through the loops and tie with a regular bow knot.
Jacket: Left side over right. The left side is on top. The right side on top means you're dead. No, really!
Belt: Hold the belt out in front of you by the middle. Walk out of the dressing room and ask someone how to tie it. They will be glad to help since someone had to show them too. No colored belts in our dojo; we only wear white belts until the day we earn the black belt. We're traditional, not commercial.
Hakama: No, we only start wearing them at black belt. So don't buy one now. Other dojos, even in our federation, may allow students to wear them before black belt, but they should not wear them at seminars.