Too many to list here's places--https://www.google.com/search?q=where+to+buy+large+stencils&safe=active&rlz=1C1CHBF_enUS750US750&source=univ&tbm=shop&tbo=u&sa=X&ved=0ahUKEwj4_OnGhYLhAhUOup4KHQVIA78QsxgILQ&biw=1366&bih=657
WalMart, JoAnn Fabrics, Michaels
Fabric, craft or hobby stores, many on line vendors, and there is always Amazon!
Try michaels crafts
Check out Stencil Revolution too.
Have you looked at Amazon?
Amazon and Michaels should have large stencils