I have heard that Brahma washed Vamana's feet with water he kept in a pot and that ultimately became Ganga. Are there other stories how Ganga originated?
Few stories are related to Ganga:
As per SRISHTI KHAND - Padma Purana (page 96), Ganga originated from Lord Vishnu's feet.
As per the chapter Vishnu and Manu of Matsya Purana, Ganga was there before the the beginning of Vaivasvata Manavantar i.e. (even when Ikshvaku was not born). But as deluge (प्रलय) happened after that so many people brought Ganga on Earth again.
As per this Chapter of Skanda Purana, Lord Brahma instructed Ganga by saying- ' Ganga! Go to the earth, where you would be known as Gomti. Sage Vashishtha will lead you to your destination. Just follow him like a daughter follows her father.'
As per the chapter The Origin of Trayambakeshwar - PART FOUR KOTI RUDRA SAMHITA of Shiva Purana, Sage Gautam requested Lord Shiva to liberate him from the sins of killing a cow. He also requested lord Shiva to manifest the stream of river Ganga that place. This Ganga became famous as Gautami Ganga.
Daughter of Himvana (sister of Parvati)
Bhagiratha brought Ganga on Earth, this Ganag is known as Bhagirathi Ganga.
As per Sambhava Parva - Mahabharata, Lord Bhrahma cursed both Mahabhisha and Ganga to take birth in Mrityu loka (Ganga in Mahabharata as wife of Shantanu)