Once you have your store up and running, then you can start making phone calls and sending emails to potential drop shipping suppliers to get approved with them.
Most novice dropshippers and sometimes even experts use platforms that help you find the more reliable and trustworthy suppliers, like SaleHoo.
You can also check Alibaba and worldwide suppliers. But for a starting drop-shipper, you better stick to the easier methods.
If you’re having trouble with that, there are some tools like Drop Ship Lifestyle 6.0, that can assist with that. Along with resources and guidelines that will make getting approved with suppliers as simple and easy as possible.