I am searching any scriptural references, even mantras, sutras, stotram etc., regarding Krishna as Dwarkanath.
Brahmanda Purana has a stotra recited by Rishi Agastya (in turn recited to him by Shesha) referring Krishna with the epithof "DvarkAnAth".
This hymn, consisting of 108 names of Vishnu is known as the Kṛṣṇapremāmṛta
Chapter 36, Upodghāta-pāda, Brahmanda Purana
अजो निरञ्जनः कामजनकः कञ्जलोचनः ।
मधुहा मथुरानाथो द्वारकानाथको बली ॥ २,३६.३१ ॥
- (51) Aja (The unborn), (52) Nīrañjana the unsullied) (53) Kāmajanaka (father of cupid) i.e. Pradyumna,. (54) Kañjalocana (the lotus-eyed), (55) Madhuhan (the slayer of demon Madhu), (56) Mathurānātha (The lord of Mathurā), (57) Dvārakānātha (the lord of Dvārakā), (58) Balin (the powerful).
There's also references in the Garga Samhita
Chapter 7, Khanda 3, Garga-Samhita
जगन्नाथो रंगनाथो द्वारकानाथ एव च ।
बद्रीनाथश्चतुष्कोणे भारतस्यापि पर्वते ॥ ३३ ॥
- On the four corners of the Govardhan hills are the four forms of the Lord viz. Jaggannath, Ranganath, Dvārakānātha, and Badrinath.