Be bold. The purpose of your website is to invite action.
Extend that invitation to Check Availability, Add to Cart, or Sign Up on each page of your website.
Your call to action should be easy to find. Make sure it has a consistent appearance and location, easily visible without scrolling down.
Placing the call to action in the header of your website can fill all these guidelines. The header is the first thing they see, so right from the start your visitors will know where to go to make a purchase.