Why not order one from home instead? A proper Japanese kimono will set you back a pretty penny, and will be tailored to Japanese tradition/fashion (not to mention sizings -is your girlfriend petite or a tall westerner? If the latter then a normal size robe may be closer to a knee-length robe on her!). There are plenty available through the internet for a lot cheaper than you can expect to pay at some overinflated tourist shop in Tokyo. A lot of them come from Thailand so are much cheaper, but still of noticable oriental design. Get your girlfriend to pick out a design she likes in advance. There are some listed here to get you started: silk kimono robe
Just be sure and use a reputable company and double check that it is proper silk you are buying - in Thailand you get people swearing on their ancestor's graves that the scarves and clothes they are selling you are silk, when actually they're a composite or satin to save money.