The flags of kings, in Ramayana and Mahabharata, were mentioned as having some symbol but not Bhagawa flag.
(i) kovidara (pomegranate) tree was mentioned on the flag of Bharata in Ramayana
एष वै सुमहान् श्रीमान् विटपी सम्प्रकाशते | विराजत्य् उद्गत स्कन्धः
कोविदार ध्वजो रथे || २-९६-१८
"The gigantic and glorious tree with superb branches is ...