You are in luck. The old methods for making curved base boards were to a) steam the wood and bend it over a form or b) to build it up from straight pieces, glue them together, and plane or machine them to the curve. Today there are flexible plastic moldings that will follow almost any curve. I don't know the manufacturer, but a source is:

Lynn Lumber Company

180 Commercial Street

Lynn, MA 01905

781-592-0400

www.lynnlumber.com

It ain't cheap, but it's cheaper than custom made hardwood.

Regards

What material are you using for base, and around what size would the radius be for the circle?

http://www.altereagle.com/

http://decks-ca.com

http://kingofcrown.com

Alter Eagle Construction & Design

I sell wood bullnose corners for radiused drywall corners if they would fit your need. distinctivewoodcraftwest.com.

Lynda