In Martinique, you will find ATMs very easily. There is even one next to our offices.
On the British islands the local currency is the Eastern Caribbean Dollar (XCD). Locally it's known as the EC dollar or ECS.
Its exchange rate is about 3 ECS for 1 Euro or 2.70 ECS for 1 US dollar.
The Euro (€) and the US Dollar (US$) are also accepted.
Visa and Mastercard are also accepted in numerous shops and businesses but not so often in smaller, local shops.
There is no point in trying to buy ECS before travelling, you won't find them in Europe. You will, however, be able to go to a Bureau de Change at the airport in Lamentin or at the harbour master's office in Martinique.
Otherwise, there are ATMs in The Grenadines (Bequia, Union...) and on Saint Lucia, (Rodney Bay and Marigot Bay) and there are also banks for carrying out exchanges, generally those near customs and immigration offices (Rodney Bay, Bequia, Union ...)