hi,
ebay would be the obvious place to check first. check completed auctions for the models you want and see what the market value is. but unfortunately ebay is more a sellers than buyers market for powerbooks, and they tend to fetch top dollar there. also be sure to check the feedback of the seller and contact them to discuss if they will guarantee the PB will work as stated. pay with Paypal and a credit card if possible (so you have some protection there as well) and dont neglect shipping insurance.
you could also try the classified section at powerbookcentral, where you might get a better price but dont have much in the way of protection against unscrupuslous sellers:
http://www.powerbookcentral.com/cgi-...ubb=forum;f=20
also check the marketplace forum here at macnn (with the same caveats)
btw, if you can stretch your budget a bit more, you might consider a refurb iBook or Al-book. the ti-books have not been known for their reliability--with bad hinge design, peeling paint, general fragility and high repair costs to boot.
refurb deals on later models are noted here:
http://www.macprices.com/refurblaptops.shtml
best wishes,
rich