Note that this guide only deals with buying kimono in Japan. When you get to go there, you may want to bring one back, either for wearing or as wall hanging, souvenir, or for using the fabric to make something else out of it.
First decide whether you want a new or a used kimono. If you opt for used, you can either play safe or cheap...
Mind that this is an expensive option - even a sale occasion costs upwards of 100,000 yen. If you don't speak any Japanese and don't have a Japanese friend to help you, your best bet is one of the large department stores where the staff is more likely to speak English than in one of the slightly cheaper, specialised boutiques. Try to catch an end of season sale - the only ones I know of for certain are mid-February and mid-September. At department stores, you can then find kimono at 5,000 yen if you're lucky - and if you like the fabric. Don't forget that you'll need various accessories which often aren't available at reduced prices. The sales floors of department stores seem to offer no consulting, while the normal (i.e. high) price departments can be expected to offer the same degree of consulting as a top-of-the-rank designer boutique in Paris.
There are two ways of buying a used kimono: the safe and the cheap way. In fact, both are cheap if compared to new kimono as the Japanese tend to throw away anything that isn't perfect anymore. (Want an anecdote? When I was a student, I've spent half a year in Japan and moved into a tiny flat. In front of my neighbour's door stood a small cooking range, standard issue, so I knocked to ask whether I could have it. Sure, said the neighbour, she had wanted to discard it - and felt compelled to warn me "But it's two years old...")
The safe way is to check out used kimono shops. They can only be found in large towns where there's a clientele made up of foreigners and those few Japanese who don't mind if the kimono has been worn once or twice. As the Japanese tend to be honest and customer-oriented, it is likely that the price asked reflects the value of the piece in question. If you discover a fault later, while you're still in Japan, I expect that you could take it back to the shop, but I've never tried. The prices asked are from 2,000 yen up. As things change so quickly in the large cities, I won't mention any address. The tourist info or a very new Lonely Planet edition may tell you about others. Or go where many foreigners go and try to pick up a copy of Tokyo Classified there.
The cheap way is to check out a flea market. As the Japanese aren't very fond of used goods, the clientele is twofold: foreigners looking for souvenirs and Japanese looking for antiques. Flea markets are only found in the largest cities, i.e. Tôkyô, Kyôto and Ôsaka. To find out about the where and when, ask Tourist Info or have a brand-new Lonely Planet ready. The one I bought mine at is at Tôgu Shrine in Harajuku, Tôkyô.
Why flea markets don't count as "safe"? Almost all the only people interested in kimono are tourists. Japanese flea market traders are the same as their colleagues around the globe: If they notice that the customer has no idea of the value of the object, they ask higher prices than the quality of the object warrants. If you later find fault with the ware, forget about taking it back. That's why.
If you are confident that you can detect wear spots, tell real silk from synthetic and good yûzen imitations from bad ones, the flea market is your best option. If you speak enough Japanese to ask a price and understand the reply, your chances are better as the seller will assume that you know a thing or two about kimono and can argue some more. Pay 1,000-2,000 yen for a simple synthetic kimono, 5,000-120,000 for painted real silk or embroidered bridal kimono, 500-1,500 for haori, 500-70,000 for obi, and not more than 500 yen for accessories such as obi-age or obi-jime. Be sure not to overlook all-black garments as they are likely to be overlooked by tourists, so they can be cheaply got even if they are pure silk. Some all-black kimono (usually men's) have linings elaborately painted and can fetch prices of 5,000-80,000 yen. The prices quoted here are what I've encountered myself in the mid-90s and do not exceed what I consider reasonable.