Sara Blakely is one of the few women who has joined the Forbes Billionaires list without help from a spouse or an inheritance. She came up with the idea for Spanx and spent two years and $5000 developing it and the $1 billion company it would become.

Blakely, then 27, moved to Atlanta, set aside her entire $5,000 savings and spent the next two years meticulously planning the launch of her product while working nine to five at Danka. She spent seven nights straight at the Georgia Tech library researching every hosiery patent ever filed. She visited craft stores like Michaels to find the right fabrics. She sought out hosiery mills in the Yellow Pages and started cold calling, only to be told no repeatedly. Immune to rejection thanks to years selling door-to-door, she decided just to show up. At the Acme-McCrary hosiery factory in Asheboro, N.C., she was turned away, only to receive a call from the manager two weeks later. He had daughters, he told her, who wouldn’t let him pass up her invention.