In the Technical page section under Tune-Up parts, part numbers are included. So you can purchase them from most Napa Auto Parts stores and several others. rockauto.com even lists Durant in their website. For 1930 they have models 614 and 617, I believe your Durant to be a model 617 if it has a 15U engine.
Where Are You From? Fremont, CA
Do You own a car built by Durant? 1931 Durant Modelm 619