Hi Christine,
I was in Sri Lanka in September, I used one of the currency counters in the airport to exchange the bulk of my currency (put on card) and then used sterling and dollars (left over from previous trips) and exchanged this at hotels as we travelled around. It is worth noting that not all hotels will exchange currency or give you the best rate.... but do not worry - the Tour Manager will be able to advise of the best opportunities for you to exchange your money.
Kindest regards,
Julia