Flex-trim. It's a resin substitute for wood. Can be made with wood grain (so it can be sort of stained) or smooth.
Alternative is having curved trim made in wood, like used to be done. About 2x cost of the plastic. Will stain up a lot nicer, though.
Casey

Here is a link that might be useful: Flex-trim

There are any number of companies that make curved door casing in various sizes, and will custom make other sizes.

The 'stock' sizes are much lower cost than a full custom.

Search for 'curved molding' and there should be plenty of hits.