Any good canvas shop can make a new cockpit cover for you by taking measurements off your boat, but the one that advertises in the DAY SAILER QUARTERLY (newsletter of the Day Sailer Association) is "The Sailor's Tailor"
http://www.sailorstailor.com/ they are in Spring Valley, OH.
That said, I have kept my DS II on a mooring every Summer since 1996 without a cover, the cockpit is "self-bailing" to a point. A small amount of water will remain in the "gutter" around the cockpit with the bailer uncorked, but rain will drain out mostly (in a heavy downpour the cockpit will not drain as fast as it fills, but it does drain fast enough to prevent swamping, leaving the 1" drainplug located on the transom out also helps in a heavy downpour). A cover will keep most of the water out and may keep things cleaner, but it does mean having to deal with a large piece of canvas each time you go sailing. If you do get a cover, be sure to use a boom crutch to support the end of the boom, the boom needs firm support to keep the cover taunt (using a topping lift or the main halyard will not allow that firm support).