While most Japanese ladies do not engage in one-night-stands, there are some who do - sometimes regularly, sometimes occasionally. Here are several options that can be considered.
Roppongi. In the Roppongi nightclubs or bars, most of the japanese girls present intend to meet a foreigner. About half of them are on the hunt for a one-night stand. The latter are easy to identify as they tend to touch the chest and arms of their potential one-night-lover early on in the conversation. It is recommended to proceed with caution with those ones as they tend to engage in one-night-stands rather often and might carry certain diseases - proper protection on all extremities should not be overlooked. Further, they are generally not very pretty and a bit overweight. The other half are "normal" japanese girls looking to meet a friend or boyfriend to practice english with. While this may seem to be a heaven for some, it should be noted that Roppongi is a competitive environment - there's as many if not more foreign gents than there are japanese ladies. Nonetheless, gentlemen in their early 20s with above-average looks do report a high success rate in this area.
Western bars. In many places in Tokyo, western-style bars can be found (as opposed to the japanese-style bars Isakaya). It's possible to meet single ladies there - the success will be more or less the same as in the west with western women.
Japanese nightclubs. Japanese don't go out dancing much, but occasionally they do and most of them won't go to Roppongi. They prefer to frequent the japanese nightclubs - as its name implies there's few foreigners there if any. Gents who fit in the visible minority category will be approached by japanese girls - but most of the time the ladies are just looking for a chat with a foreigner and nothing else. Sometimes, it happens that they want more. There's several japanese nightclubs in Shibuya and some can be found scattered here and there throughout Tokyo. These establishments can be identified easily as the cover charge is 3000 yens (for the foreigner nightclubs it's 1000 yens). The drinks are cheaper thus.
Nampa. This is a game that young japanese men play on the streets. It consists of approaching women walking by and asking them to go for a cup of coffee or a one-hour session at the karaoke box or something similar. This is lots of fun but involves lots of rejection. For the gents who can speak a bit of japanese and who have good looks/abilities, the success rate will be around 1%. Seems like a lot of work, and yes - it is. Should the lady agree to follow the gent to the coffee shop or the karaoke, there's good chances a one night stand can occur on the same day or on a subsequent meeting as agreed by both parties. Here are some good spots for nampa: Shibuya station and on or nearby the campuses of Aoyama-Gaukin, Gakushuin, Shirayuri, Seikei, and Sophia universities. The university campuses just mentioned consist mostly of female students and many of them come from a wealthy milieu. While they don't rank too highly on the intellect scale, they generally speak english more fluently than the average.
In all cases, should there be a substantial age difference in the gentleman's favour, an adequate gift is sometimes necessary to charm the lady. It seems that 10000-20000 yens per night is sufficient when the age gap is of 10-20 years.