How to get Area or specific location boundaries using flutter map? (With Flutter Map only , not in GOOGLE MAPS)

Question

I need to show area boundaries in my flutter map like in Google Maps. If I click or search any area or city I need to show the boundaries of that area or city how to do it. I attached a sample image for reference.Area image

I have searched my sites none of the results are not satisfied with my output. Where to get the area service boundaries.