Zappos is the best place. Free next day shipping and free returns. They have a good selection.
Crocs.com is okay, but shipping is rarely free and the prices are higher than Zappos for most styles. If you can score a sale AND free shipping on Crocs, that's the best way to buy directly.
My boys have worn crocs almost exclusively for the last 15 years, so I've had a LOT of experience buying them.