it isn't so much where you buy it from, as much as it iss what you get.
samsung, kingston, crucial the brands are all fine.
I personally don't buy into the crucial hype. That is all my work buys and i see plenty of sticks doa or get flakey, just like every other brand.
i bought some cheap ram from macsale.com the owc house brand, and it worked fine in my pismo.
I would hit pricewatch.com and search for the best deal on a brand name ram that meets the specs of your powerbook.