The best way to learn which buses go to where from Istanbul is to go to the site of Central Bus Station;http://www.istanbul.net.tr/istanbul_otogar.asp .
Write your destination city/town in the box you will see in this page and you will get a list of the companies and their phone numbers which go to this town.
Phone and Address of Istanbul Central Bus Station (Otogar) :
Büyük Istanbul Otogari
Telefon: 212 658 05 05 – 658 10 10
Sefer Yerleri:Faks: +90.212 658 28 58
BAYRAMPASA 34220 ISTANBUL
Major Bus Companies :
Has Turizm : http://www.hasturizm.com.tr
Nilüfer Turizm : http://www.niluferturizm.com.tr (online tickets available)
Ulusoy : http://www.ulusoy.com.tr/ (online tickets available)
Varan : http://www.varan.com.tr(online tickets available)
Pamukkale : http://www.pamukkaleturizm.com.tr(online tickets available)
Kamil Koç : http://www.kamilkoc.com.tr(online tickets available)
Metro Turizm : http://www.metroturizm.com.tr(online tickets available)
Uludağ Turizm : http://www.uludagturizm.com.tr
AIRLINES & AIRPORTS
A list of the airline company offices: http://www.istanbul.net.tr/istanbul_kent_rehberi_altgrup.asp?ustgrupid=8&altgrupid=21
Airports of Istanbul:
Sabiha Gökçen : http://www.sgairport.com/ Sabiha Airport is on the Asian side of the city about 60 kilometers away, with very poor connections to the city center.
Ataturk Airport : http://www.ataturkairport.com/tr/homepage.htm
HAVAS AIRPORT SERVICE BUSES Timetable and Stations :http://www.havas.com.tr/en/otobus_istanbul.asp
Please notice that Havas does not serve to Taksim with shuttle buses from Sabiha Gokçen Airport. However there is a private shuttle service of Corendon airlines from this airport at certain hours of the day and night to Taksim square.
Major National Carriers:
Turkish Airlines : http://www.thy.com/en/index.php
http://www4.thy.com/troyaonline/schedule.tk online tickets
Pegasus Airlines : http://www.flypgs.com/eng/index.asp (online tickets available)
SKY Air : http://www.skyairlines.net/ (online tickets available)
Onur Air : http://www.onurair.com.tr/ (online tickets available)
Atlas Jet : http://www.atlasjet.com/ (online tickets available)
the following airlines are just either doing cargo, or not functioning at the moment:
Fly Air: http://www.flyair.com.tr/
Free Bird Airlines : http://www.freebirdairlines.com/
Inter Air : http://www.interekspres.com/
KTHY (Cyprus air) : http://www.kthy.net/
Kuzu Air: http://www.kuzuairlines.com/
MNG Airlines : http://www.mngaircargo.com/
Saga Air : http://www.sagaairlines.com/TR/
Corendon : http://www.corendon.com/english/HomeSearch.asp
RAILWAY
http://www.tcdd.gov.tr Turkish Railways...