Is Ucchista ganapati mentioned in Puranas? Is the story of Ucchista ganapati mentioned in scriptures?
32 forms of Lord Ganesha are mentioned in Mudgala Purana one among which is Ucchishtha Ganapati.
From this article (which names the 32 forms and their Dhyana Shlokas) on Hindupedia.com:
4.Pingala (Sidhi ) Ganapathi (The occult Ganesa)
Pakva chootha kalpa manjarim ikshu danda thila modhakai saha ,
Udwahan parasu hastha they Nama sri samrudhipatha deva Pingala.
Salutations to the reddish brown coloured god armed with axe in his hand , who signifies plenty , Who holds ripe mango , bouquet of turmeric , sugarcane and sesame sweet.
5. Uchishta Ganapathi (The Ganesa who has left remnants)
Neelabjam dadimi veena saali gunjaksha suthrakam,
Dadadad uchishta naamaayam Ganesa pathu mokshadha
Let The Ganapathi called the Ganapathi who gives the left over. Who holds a blue lotus, Pomegranate, Veena , rosary
The linked article is mentioning the Dhyana Slokas for all the 32 forms of Lord Ganesha mentioned in the Mudgala Purana.
According to Mudgala Purana , a book dealing with Lord Ganesa, there are thirty two forms of Ganesa. These are listed described along with pictures and Dhyana Slokas are in a Kannada book called Sri Thathwa nidhi, in the chapter on Shiva Nidhi. These pictures along with the slokas are sculpturally represented along with Dhyana Slokas in the temple at Nanjangud as well as Chamaraja nagar near Mysore
This answer provides some more details about the Tantric method of worshipping this particular form of Ganapati.
Also, Mudgala Purana, referred to in the Hindupedia article, is a minor Purana (Upapurana).
The Mudgala Purana (Sanskrit:मुद्गल पुराणम्; mudgala purāṇam) is a Hindu religious text dedicated to the Hindu deity Ganesha (Gaṇeśa). It is an upapurāṇa that includes many stories and ritualistic elements relating to Ganesha. The Ganesha Purana and the Mudgala Purana are core scriptures for devotees of Ganesha, known as Ganapatyas (Gāṇapatya). These are the only two Purana that are exclusively dedicated to Ganesha.