Be bold. The entire purpose of building a website is to invite visitors to act. Extend the invitation to Check Availability, Add to Cart, or Sign Up on each page of your website. Make sure your call to action is placed consistently and prominently.
Consistency means the call to action uses the same format throughout the website and is located in the same spot on each page.
To be prominent, it needs to be easy to find, preferably above the fold so it's visible without scrolling down.
Placing the call to action in the header of the website fills all these requirements. It’s a lot like putting the checkout line at the front of the store. The header is the first thing they see, so right from the start your visitors will know where to go to make their purchase.