Never change your money to Thai Baht before you leave your country of residence, you will get a far better exchange rate in Thailand. There are exchange booths at the airport, just change enough to get you to your hotel and change the rest at one of the many exchange booths. This is perfectly safe as all the Thai booths are operated by Banks.

You will not have a problem finding an ATM in Phuket, they are everywhere, (including the airport) and also in most 7/11 shops. All the ATM'S have instructions in English and the maximum withdrawal per day is 20,000 baht, except for the Bank of Ayutthaya which has a daily limit of 30,000baht, Bangkok Bank 25,000baht and Krungsri up to 30,000baht.  Of course this also depends on you own banks daily limit.

Look for stickers displaying Maestro, Cirrus or Plus on the machines and if it's on your card you will not have a problem. We use Westpac and National Bank and have not found a machine yet that we cannot draw money from. There is a 150baht charge for using foreign Visa cards and 180baht for Mastercard in Thai atm's.

 You will need to take your passport with you to cash Travelers cheques.

The link below will give you the exact up to the minute rates from all banks in Thailand for changing cash, travelers cheques and withdrawing cash from atm machines.

Foreign Currency Exchange Rates - Day to Day Data



Be extremely careful when using credit cards in Phuket or anywhere for that matter.

Five weeks after we returned home one year my sister had a call from her bank saying her credit card was used in Malaysia on the weekend. She only used her card twice in Phuket, once at the duty free shop at Phuket airport and also on checking out of Andaman Beach Suites. She also used it once at the duty free shop in Kuala Lumpur airport.

The card never left her sight each time.

A little update on this tip, friends who were also with us on this holiday have just found out their credit card has also been used. They also only used their credit card at the same places listed above.
So just be aware that these things can happen and be very careful. 

Nowadays it doesn't even matter if the card does not leave your sight as the fraud happens via the telephone line when the card is swiped in front of you.  If you want to go into an actual bank to make a cash withdrawal from a credit card instead of using an atm there is a link below with the addresses of some banks.