Hmm, depending on what kind of bikini wax you require (brazillian, hollywood or the thigh side burns variety!) I would reccomend taking a turkish speaker with you, the first time at least. Turks are very much in favour of the EVERYTHING off look, and will be quite disgusted with any other arrangement!
Guzellik salonu's are OK and cheap (I go to one in Moda), but from what I can see they are not properly trained and their technique is verging on gynaecological assault. When I get my legs done there's one girl per leg, it's fricking agony but very quick.
I believe that you can get agda at hammam's too.
I reckon your best bet is the californian nail bar in Nisantasi, its owned by an american lady so you should be able to negotiate more easily as they are used to foreigners. I've only ever had manis and pedis there (which are excellent provided you let them know what you want, you get a much more westernified treatment) so cant comment on the waxing but you can email them in advance and specify exactly what you want, so that you don't have a turkish meltdown when you get there.
http://www.californianailbar.com, I'll pm you the owners email. Can you receive pm's yet?