Lakshmi is seated on Vishnu's chest and is called विष्णुवक्षस्स्थलस्थिता (Residing in Vishnu’s Chest)
The supreme Goddess is seated on Sadasiva per this post;
Who are Pancha brahmas' Lalita Devi sits on and Why are they called so?
Are these the only examples in scripture?
Sri Kālika Upa-Purana Chapter 58.:
विष्णुब्रह्माशिवैरदेवै ध्रियते सा जगन्मयी।
सितप्रेतो महादेवो ब्रह्मा लोहित पङ्कजं।। 65
This Jaganmayi Devi Shivā is always borne by Vishnu, Brahma and Shiva; for Mahadeva is the preta and Brahma is pink lotus.
हरिर्हरिस्तु विज्ञेयो वाहनानि मनोजसः।
स्वमूर्त्या वाहनत्वन्तु तेषषा यस्मान्न युज्यते ।। 66
Hari is lion, and these Tridevas serve as Vahanas of the illustrious Devi. However, the three of them cannot attain Devi's Vahanatva in their individual forms.
I hope this helps.
