What are some non-standard sizes for drain caps?

Try contacting a hardware store representative to find out what other sizes stores offer. Browsing online hardware stores may yield more results for you than perusing your local stores. Online hardware stores may have access to a wider selection of general equipment and maintenance products, which will improve your chances of finding what you are looking for. If you want, check out http://www.bicwarehouse.com to talk to a customer service representative who can steer you in the right direction.