James Kopp makes a bass dulcian reed. (koppreeds.com) I haven't tried it and have only played the Tenor Dulcian myself. Someone told me that a tenor dulcian could work ok with a standard bassoon reed and a bass dulcian with a contra reed. I might just be making that up, though. I took an old bassoon reed and cut it down a bit and did a sort of oboe scrape to it. Sounded terrible, but it worked. Take some old junky reeds and just experiment a bit.
The Early Music Shoppe in the U.K. might be a better bet for you though. Their website is www.e-m-s.com Again, I have not tried their products, I'm only telling you what I have found. Also try contacting Guntram Wolf in Germany. He specializes in making historical instruments and might have some wonderful products and information for you. www.guntramwolf.de (English and Deutsch versions available)
M.M.A., D.M.A. University of Illinois at Urbana/Champaign: B.Mus. Lawrence University
Bassoon professor at University of Wisconsin Eau-Claire
Maker of the Little-Jake electric bassoon pickup and Weasel bassoon reeds