It depends on the price that you are willing to pay. If you can afford it, go for the Macbook.
If you can only spend $4-500, then a late-model Powerbook would be a good buy for you. As a bonus not available on the Macbook you will get backlit keyboard (which I love on my PB), FW800 port (which I use occasionally), as well as built-in modem (which I never used in almost 4 years, but some people might find it a plus). Also, you will avoid the glossy screen (which is the main reason why I personally might have gone with PB if I were at your place).
As for the best place to get one, check your local craigslist - you have the best chance of finding the best prices there. You might also check some online stores specializing in used macs (google: used powerbook), but those carry higher price tags.