Ken, you can buy it here (location is England, but he accepts credit cards):
http://www.earlygaelicharp.info/emporium/books/carolan.htm
My suggestion is to go for the latest edition of Donal O’Sullivan’s biography, in one volume (black cover, second title down). It contains every word of the original two volumes, plus an appendix I wrote concerning some newly-discovered material from a manuscript in Scotland, and some other updates. (The first edition is about 60-odd years old.)
If that link doesn’t work you can email an enquiry to simon@simonchadwick.net
Dr. O’Sullivan’s work has not been bettered, and you’ll probably also like some of the amusing anecdotes that Carolan and other harpers got up to. They make great stage chat!