Despite bordering countries which have the richest oil resources, fuel in Turkey is ridiculously expensive, in fact one of the most expensive in the world because of the very heavy taxes. For example, a litre of unleaded gasoline costs more than 4.00 TL (~€ 1.80/~US$ 2.40, that makes ~US$ 9.60 per gallon!). Diesel and LPG is less damaging to your wallet (and to the environment in case of LPG), but not that drastically.
Petrol stations (benzin istasyonu) are frequently lined along the highways, most (if not all) serving round the clock and accepting credit cards (you have to get out of the car and enter the station building to enter your PIN code if you are using credit card). In all of them you can find unleaded gasoline (kurşunsuz), diesel (dizel or motorin), and LPG (liquid petroleum gas, LPG). In many (if not most) of them you can also find CNG (compressed natural gas, CNG). Though, petrol stations in the villages off the beaten track are exception, all they offer is often limited to only diesel, which is used for running the agricultural machinery. It is advised to keep the gas tank full if you are going to stray away from main roads. Also petrol stations along the motorways (toll-ways) are rarer than other highways, usually once every 40-50 kms. Make sure to fill your tank in the first station you’ll pass by (there are signs indicating you are soon going to pass by one) if your “tank is getting empty” alert signal is on.
Biofuels are not common. What most resembles a biofuel available to a casual driver is sold in some of the stations affiliated with national chain Petrol Ofisi under the name biyobenzin. But still it is not mostly biofuel at all – it consists of a little bioethanol (2% of the total volume) stirred into pure gasoline which makes up the rest (98%). Biodiesel is in an experimental stage yet, not available in the market.