two that spring to mind are http://agphotographic.co.ukandsilverprint.co.uk both sell 75 watt enlarging bulbs, which if my memory serves is the correct bulb for the durst c 35, I used to have one before I got into MF, and I think they are screw fit bulbs. You can also try Jessops if you have one local, as they also stock them, according to their website, http://jessops.co.uk
Richard