If you’re using Amazon, you go to Amazon, search for a bike rack, choose a bike rack, and order it with one click.
If you’re not using Amazon and you don’t know where to buy a bike rack, you go to Google/Bing/DuckDuckGo/etc, search “buy bike rack online”, scroll through the results, opening website after website until you find what you’re looking for, add the item to your cart, go to checkout, enter your name/address/card info/PayPal/etc, and submit.