The term Payment Source in CrystalCommerce, pertains to the integrated services in your admin that allows your customers to directly pay for products using their debit or credit cards on your ecommerce website. These two integrated services found in the Account > Payments tab are Authorize.net or Beanstream.
Having a payment source is NOT mandatory. If you do not want to have an Authorize.net account or a Beanstream account when you launch your store, you can still launch by simply having a Paypal Express or Paypal Standard integrated in your admin to start receiving payments. As your store grows, you can contact a vendor that would sell an Authorize.net or a Beanstream account and pay for the transaction fees from these payment sources.
So what is mandatory when I am launching my online store? You need to make sure your customers are able to pay for the products you have made available using your CrystalCommerce inventory. Whether it is Paypal, Authorize.net, or Beanstream is up to your preference.
Here are some helpful articles on how to setup different payment methods on your website: