Money in Istanbul
With hardly a town lacking an ATM, it's perfectly possible to get around Turkey with nothing else but a credit or debit card. Just remember to draw out money in the towns to tide you through the villages and keep some cash in reserve for the inevitable day when the machine throws a wobbly.
Changing Your Money
It's easy to change major currencies in exchange offices, and many post offices (PTTs), shops and hotels; however, banks may make heavier weather of it. Cashing even major travellers cheques can be a hassle (although post offices in tourist areas are a good bet) and the exchange rate is usually slightly lower. Places that don't charge a commission usually offer a worse exchange rate instead.
Although Turkey has no black market, foreign currencies are readily accepted in shops, hotels and restaurants in many tourist areas. If you're left high and dry, most banks countrywide can do Western Union transfers.
Also read Currency in Turkey for more information.
In cheaper restaurants it's not necessary to leave more than a few coins in the change plate. In more expensive restaurants, tipping is customary. Even if a 10-15% service charge is added to your bill, you're expected to give around 5% to the waiter directly and perhaps the same amount to the maitre d'. Porters expect a dollar or so; in taxis you might like to round up the bill; in other situations, for example, helpful guardians at archaeological sites, delicacy is required. Although a tip may be initially refused through politeness, you should offer the money a second and third time. After three refusals, you can safely assume they really don't want the money. In hamams you should tip 10% to 20% of the admission price to the masseuse/masseur.
Insider Note :
There is a common misknowledge about Turkey : "Always bargain before you buy something" ... Well, that works for small shops in Grand Bazaar, gift shops, etc. Mainly if you are buying something in a tourist attraction place, always bargain... But, please don't make this a habit, as you wont be taken serious in big supermarkets, public transport and luxury restaurants...