Almost any NPN transistor (2N3904, 2N2222, BC537, etc) will do. Large ones are ok too. (TIP31, TIP121, etc.) 1k ohms from PIR output to base; Emitter to 0V; collector to buzzer (or relay if you prefer). You would like a small (1N914/1N4148) diode across the buzzer/relay; cathode on collector, anode to +V . Again, a big diode is OK (e.g. 1N4001 thru 1N4007 etc.)
[edit] The reason I chose 1k is that the app note for the PIR suggested 330 as a series resistor for a test LED. 1k should provide plenty of current (2-3 mA) for a typical transistor (gain 50 or better) to turn on. [/edit]