Crucial or Samsung RAM. Newegg.com for Crucial RAM, SVC.com for Samsung RAM. Reliable and trustworthy retailors. Check resellerratings.com if you never heard of them.
Apple totally overcharges for the Samsung RAM they use in their system because most Mac buyers are gullible. Wait, let me rephrase that. They rather have Apple do the less than a minute RAM installation for them, and are happy to pay the $200 labor.
Original Samsung PC3200 RAM: 256MB/$60, 512MB/$105
Original Crucial PC3200 RAM: 256MB/$50, 512MB/$93