One of the easiest things to do might be to find a wall you like, find out who owns it, put together a portfolio that shows your experience, and ask them.
Murals on Public Property / City Owned Walls? (Collegetown Garage, Green St Garage, Seneca St Garage, City Hall, DPW buildings, Stairwells, Retaining Walls, Parks, bridges, etc)
Application to paint on city property: http://www.cityofithaca.org/DocumentCenter/Home/View/1458 If possible, overlay a sketch (painted or drawn) of your proposal on a photograph of the blank wall. Ignore the dates listed on the application & just get your application in ASAP. Send applications to firstname.lastname@example.org and email@example.com