You cannot export Tunisian currency, and for that reason your bank cannot order any for you to take with you. Most tourists arrive with no currency - it's easy enough to obtain it.

The exchange rate is fixed by the Government, and you will be offered that rate at the airport and at your hotel. You may find it better to exchange some currency at these locations rather than use ATMs, nearly all banks and credit cards place huge surcharges on overseas transactions.  (Suggestion: Get yourself a  Halifax Clarity credit card  It can be used in foreigh banks and ATMs and does not charge any %, unlike almost all other cards for changing money - you get the straight exchange rate.   Halifax Clarity card is unique in this respect. It has no foreign loading however to take full advantage of this if you want to obtain cash, you have to pre-load your card with credit before you go otherwise you will be charged interest. 1 Tunisian Dinar (TD) is divided into a thousand, so 15 dinars will be written 15,000. As at December 2006, 1TD = 42p approx, making approx 2.4 TD to the GB pound. March 2011 update, not changed much 1 GBP in currently 2.25 TD or 1 TD is 44p.

Again, note that it is illegal to take ANY Tunisian currency out of the country. You must change back ALL currency**, including coins, when you leave. You can still make purchases at the airside shops and cafes, since they take a range of non-Tunisian currencies, notably Euro, GBP and USD.  

**The Tunisian authorities have the right to search your baggage and spot-searches are common. They really do mean it - NO currency is to be exported.

Before leaving the country you should contact your bank and let them know where you're going to and for how long, otherwise you could have your card(s) blocked due to irregular spending patterns.  However, a lot of banks just ignore this, so make sure you've got your bank's telephone number written down - you may need it!