Do you have a quarry nearby? Might be a good source as stone will be local and will look like it belongs to your area. Buying stone at some of the large GCs can be costly. A quarry might be cheaper depending on what you are looking for.
Do you mean you want to raise the level of the soil where the tree is? If so, that could cause damage to the tree.

You can buy attractive terracotta edging tiles which would look OK. Nothing wrong with sleepers either. I would worry about the raised soil area round the tree though - doesn't do them any good.

Rockeries look best with fewer bigger rocks, with at least half the rock buried. The idea is to try to imitate natural outcrops. A few stones laid on the surface never look convincing. Quite honestly I think that area would look better as a level bed and it would be better for the tree. You can still sink some big rocks into it, possibly even use those as your edging.

If you are intending to cover the surface with small stones try builders yards if you haven't got a nearby quarry- they often have a selection. As its such a small area it might be good to match whatever stone you use to the shingle you already have nearby. Before you decide have a look at the ideas on here. Click on R at the bottom of this page and then choose Rockery ideas t see what some of our members have done.

Your tree is a deciduous on and will shed leaves over any planting. As already said you will damage the tree (kill it) if you try to bury any part of the trunk in soil. Why not use woodland plants and bulbs to give you Spring colour. If you could use the soil area as pathway you could build up a bed against the wall. Old sleepers or stones will be fine. Its hard to see how big the area is but you could circle the tree with a seat. In your new raised bed you could grow herbaceous perennials and Spring flowering bulbs. The tree leaves will not cause them problems.

Lovely idea Scotsgran!

Thank you Steragram. I love to see how members plan out their gardens.