You can look after websites like Aliexpress, Sammydress, Dhgate etc., for your niche products
Using Dropshipmate you can import your niche products into your shopify store with an ease. With this app you can import unlimited products and can also enable Auto Update feature.
All the Best...
Here are variants of Maxi Dresses for dropshipping https://market.dropwow.com/womens-clothing-and-accessories/dresses/