This is a subject that has come up before, but I've run across a couple of new possibilities for those who do not wish to spend a mint on tags.

I was at Lowes a few days ago picking up a new vertical blind for a patio door.  While there I saw the clerk shortening a faux-wooden set of venetian blinds.  I came home with about fifty little chunks of plastic. 

Think about it.  The materiel is designed to live in the sunlight.  It can take paint, engraving or permanent marker.  It can be worked like wood, so drilling the holes to nail it to a post or hang it on a tree is not a problem.  The negative side is that the faux-wood blinds are expensive and when  cut to size the extra pieces are short, so you'd have to find one that is being junked or buy one.  Look in the thrift stores.

When I took my old blind down I saved the individual slats.  They too are plastic, have lived in the sun on the E. side of my house for fifteen years, and had my grands not punched holes in the slats and jerked out a couple of them they would still be hanging.  Today I cut one of them into tag-sized pieces, tried a permanent marker and a paint stick.  Punched a hole in one with my paper punch.  I know the stuff will engrave as well as light aluminum.   The stuff can be cut with heavy scissors and a nylon cord would hang it in the tree decades.  The slats are three and a half inches wide, so tag size is going to be a matter of choice. 

Just an idea for those of you who improvise. 
Ox