The Watertown Tree Warden has a variety of tree species available for planting; contact Chris Hayward at chayward@watertown-ma.gov or (617) 972-6426. He will come out to your site to see if there is a suitable planting location, whether in the street strip or even on your private property, within 20-feet of the public right of way. 

New efforts, see: Speaks For The Trees, founded by Watertown resident and member of Trees For Watertown!