Many wood shops can produce exterior and interior trim for oval, and other shaped windows. Call around to some local lumberyards, as they should either offer the products, or be able to refer you to someone.
Another option would be to use composite trim, like Azek or ATW. It's a PVC trim made specifically for your window. I've used the ATW products before with good results.
But trim is absolutely available.

Here is a link that might be useful: ATW

Usually any time you order an arched top window of any kind, you can get it with pre-made trim for that window. Go back to the manufacturer and see if you can get the trim you want.

As for the exterior trim, I'm confused on your question. When you speak of trim, do you mean the 2" wood brickmould that goes around most wood windows or do you have an aluminum clad window with a nail fin? If your replacing an existing window that used to have a wooden brickmould with an aluminum clad window with nail fin, your exterior void where the old brickmould was located must be custom bent on a brake and capped off. It is done by piecing to get the arch. Unfortunately this is the only way to do it. A good skilled installer can do this fairly easily. Otherwise a wood window should come with brickmould already attached to the exterior side. Hope this was in the right direction for you!