There are different kinds of kimono, and the more formal you go (usually) the more you need to wear them. So it depends a lot on what KIND of kimono you're getting, but the basics are mostly the same.
Check here and see which of these kimono you're interested in buying:
http://www.wafuku.co.uk/kimonoinfo12.htmBut in general, you'll definitely need the naga-juban OR get just a tsuke-eri or kantan-eri, a piece that is just the collar that can be worn under the kimono to give the impression of wearing a naga-juban. However the nagajuban is better usually.
You'll also need the obi-age, which is a long piece of fabric that holds the padding used to make the obi tie in the back. This is often sold as a set with the obi-jime, the "string" in the middle of the obi. BTW this is not exactly used to tie the obi... the obi is tied by itself, but depending on the style of knot in the back, the obi-age and obi-jime help hold it all together and secure it. You need both, unless you're wearing a yukata or summer komon that uses a smaller-sized obi and therefore don't need the other pieces. Note there are different types of obi as well depending on the kimono. The obi usually matches the formality of the kimono, so it's important to get the correct obi for the kimono you want.
Do you know anything about wearing the kimono or tying the obi? To be honest, it isn't easy for a beginner and can take lots of practice. If it is your first time buying a kimono or accessories, I recommend to start with a yukata (it's the easiest to learn, and also the cheapest usually), and to get a beginner's set for the under stuff and ties. Even for a yukata, for me I use 7-10 separate pieces to wear it. Honestly, I recommend getting a set in order to wear the more formal pieces:
http://global.rakuten.com/en/store/darumaya/item/ws069m/ All of these pieces are used to tie the kimono together correctly.
For places, I've bought from Ichiroya before:
http://www.ichiroya.com I've also bought from Yamatoku, but they don't seem to be around anymore.
But I'd really consider getting a full set with everything necessary included if this is your first kimono purchase. Even for someone who wears kimono regularly, it can be a bit confusing to make sure you've got all the pieces necessary. Check out rakuten for some good deals.