* Using a similar-sounding name: You could choose a Greek name with a similar sound or feel, such as Ισαβέλλα (Isavella). This is a common transliteration and is probably the closest you'll get.
* Translating the meaning: The name Isabella is of various origins (possibly Germanic or Spanish) and means "my God is abundance" or variations thereof. You could find a Greek name that reflects this meaning, though it would be a semantic translation rather than a phonetic one. Finding a perfect equivalent is difficult, but names related to abundance or God's grace could be considered.
Therefore, Ισαβέλλα (Isavella) is the most practical and common way to represent Isabella in Greek.