Chantress is available in the US in hardcover and paperback. You can buy it from your local bookstore or from the following sites.
Chantress is also available in Canada.
You can find Chantress in the UK at Amazon.co.uk, Foyles, The Book Depository, or at the wonderful indie bookshop Mostly Books in Abingdon.
You can get audio books of the Chantress trilogy, read by the amazing Mary Jane Wells, on Audible.com and in the UK on Audible.co.uk.