Alibaba, Amazon, Ebay, Aliexpress etc., you can refer to those websites where you can find USA dropshippers for surfing accessories. To import products into your shopify store, I refer Dropshipmate shopify app which helps you. With a single-click you can directly import the products and also Product Reviews too. This app specifies in it and also Daily Auto Update/Sync
All the Best...
I am also planning to work like you, it is an interesting business. I have checked but I am not sure till now how will be better to import(using what apps). I want to open a online shop with surfing products, paddle boarding, wetsuits and so on... maybe to create a guide about how to stand up paddle and other useful things about beginners ( all together products and info ). I think it is a good idea which will bring money and it will be useful for people.