Well, I am not sure how old you should go. I personally would not go back any further than a wallstreet model.
How much can you spend? You can usually find a wallstreet model PB for around $200-300, depending on specs. You may also be able to find a good deal on an old (clamshell) iBook. Maybe a Lombard model PB can be found for not too much more also...
The good news with the WS/Lombard is that they can both be upgraded to pretty capable machines and I believe all the models I have listed will run os X with enough Ram. Definitely os 9.
would not recommend Kanga model PB or older...
As for where to look...LowEndMac is a good start, so is PowerbookCentral and eBay. It might also be useful to snoop around at any local retail store selling macs for trade-ins etc. You might also try craigslist.com, if they have your city (not sure where you live in Canada)
prices I mentioned are in US$, good luck.