I bought mine from Powells:-
http://www.powells.com/biblio/1-9780943396583-1
But I see it is on sale at many places, and cheaper than Powells, eg, directly from the publisher:-
http://www.willbell.com/handbook/nitesky.htm
If you can get free postage, then with the current exchange rate you may get all three for close to $100 - a bargain!