I can't find the download links for Craft Pro. Where are they?
Douglas's answer is correct... Also, if you'd simply like to try Craft Pro before purchasing it, you can do so while running Craft in your local environment.
It's now possible to try Craft Pro from any non-public domain:
Assuming it is already installed and running, there should be a link to upgrade in the footer of the control panel which generates a modal window with options to buy or try. Keep in mind you will need to be running your install on a domain that ends in 'craft.dev' (i.e. mysite.craft.dev). From the docs:
Craft won’t automatically upgrade itself; you will still have to do that manually. Click on the upgrade link in the footer of the Craft Control Panel, and you will find new “Test” buttons beside each of the buy buttons:
Otherwise you can download craft from anywhere on the craftcms site (link is in the top-right).