Overstock does not allow dropshipping.
Support for 1-click product importing has been added to overstock.com based on user requests since some store owners want to drop ship products from those sites with 3rd party warehouse.
You can read more info about 3rd party warehouse services here -> https://www.shopify.com/blog/60645957-everything-you-ever-wanted-to-know-about-third-party-warehousing-and-fulfillment
And some recommendations for warehouses here -> https://fitsmallbusiness.com/best-3pl-companies-third-party-logistics/
If you want to focus on direct dropshipping I would recommend using Chinese wholesalers like Aliexpress, Banggood, Sammydress