Does anyone know somewhere to buy stencils in Kunming (Roman alphabet, not Chinese characters)?

Making your own may be an option, depends on size.
www.spraypaintstencils.com/alphabetstencils/stencilindex.html
At the top of the page are the ones you purchase, at the bottom of the page are the free ones.
I am sure there are more online.

Thanks tigertiger, if somewhere in Kunming has them, we'd prefer that over cutting them all out (may just freehand paint it over doing that), anyone else know a place?

The only other option, might be the art shops. but if it is a spray paint stencil, then unlikely.

If it is for wooden shipping crates, I have just printed what I wanted on A4 paper and glued this on to the box.

It might help a little if we knew what you wanted them for.