Visas: Visas are required for many visitors, when obtaining visas  at the border or airport, seek out the separate visa sales desk before getting in the line for passport control. UK passport holders need a visa. In the past you used to be able to purchase this on arrival but from 10th April 2014 they will only be available purchased in advance online. Here is the link: Official Turkish Visa site

For UK passport holders, the visa is valid for 3 months, so if you're planning regular vists to Turkey you won't necessarily need to pay each visit

For American or Canadian tourists, the Visa is also valid for 3 months.  For now you can buy them in person at İstanbul Atatürk Airport, you MUST pay with cash or credit card.  Canadian dollars are NOT accepted.  The cost is 60USD.  It is recommended to bring 60USD cash with you as also some credit cards may not work here. There are no ATMs before this point so it is best to be prepared! Remember after 10th April 2014 they will only be available online.