It's great, and its tie-in with the movie has led to it enjoying a reissue
on mass-market paperback two weeks ago. Don't know if that helps you depending on where you live, but it's definitely widely available and cheap.
Incidentally, I'm betting the similarities to the Disney film aren't going to be much greater than those with the Monkey Island games... the impression I get is that after choosing to use Blackbeard and the Fountain of Youth as integral story elements, the studio optioned Powers' novel just to avoid a potential legal ruckus. The Pirates' screenwriters have actually been citing On Stranger Tides as an influence on the series since the first installment, so there's a part of me that's thinking this is just an explicit curtsy to the book for what the series owes it in general.