I have found a very local supplier for my t-shirts dropshipping business. This supplier is already working with dropshipping bbut within a local level so they aren't on shopify/oberlo/etc.
I think I'm missing a very easy step, but how do I add their product on my shopify account and make sure that whenever someone bus from my website, the supplier gets notified to produce that specific product?
You have a choice to do it manually. Contact to the supplier and ask him to give the product details/data or csv file so that you can add that product into your shopify store through csv file format. Shopify allows to import products through csv files.
I suggest to check out Dropshipmate app which hleps you to import products from 18+ websites.
All the Best...