I buy it from their website. Unfortunately Wilson's is located in Canada, so shipping is a bit steep. I buy five pounds at a time (lasts me a year.)
You may be able to find it at a health food store, which would be advantageous if you want a small quantity. I brew a third of a cup of ground coffee in about 40 ounces of water. Wilson's is very easy to retain.
I'd be happy to answer any other questions you may have.