Definitely follow Steven's advice, and treat it as a convertible. Make sure it doesn't get soapy either, or you'll foam away, driving down the road in a rain storm.
Haartz Sonnendeck was most Cali Roof's OEM material. BMW had stretching problems in latter half of the 1990's and switched to Haartz Stafast.
There's a few places that sell that material in the form of replacement tops for cabrios. I have not seen anybody except BMW sell replacement fabric for the California top though.