Question
We would like to have the scalloped stone edging that they use for tree rings but
can only find the 24 inch long ones.  When they are put together they go way
beyond the 24" across space that we want it to be.  Do you have any
suggestions for something else we could use to have a pretty border that is
circular instead of straight edges?

Answer
landscape edging comes in SOOoo many varieties.  I personally think tree rings are not healthy for existing trees.  Some species don't seem to mind, while others can develop all sorts of problems - even die (certain old, established oaks and hardwoods).

Since it is only 24 inches, the tree is probably rather small or newly planted, so it should acclimate better.

To your edging options: scalloped stone only comes in one size.  other options are to use recycled "plastic" wood or metal edging that bends, or a custom-pour concrete mow curb, which isn't that expensive.

Here are extruded concrete examples:
http://www.abetteredge.com/
www.reedescustomcurb.com/

Of course, these folks are in Colorado/Wisconsin, so it depends on what is available in your area. There are MANY other franchises for curbing, and lots of different styles.

If you just want to stick something simple in the ground, here is a neat product:  http://www.edgn.com.au/garden_edging_installation.html

All the best ~M