The speedometer light and the light socket were missing from the 1990 750 Sport that I recently purchased. I was able to get a replacement socket and bulb from egauges.com. http://www.egauges.com
I have bought items from this site many times, and the parts have arrived quickly and have been good quality.
Go to http://www.egauges.com and look in the accessories tab under bulbs and sockets. Then look for VDO Insulated Light Bulb Sockets. I ordered Part No: 600-819. Bulbs can be purchased there are locally. Here is the contact information.
Contact Information for the site.
Phone: 1-800-303-6211 (517-625-1884 outside the U.S.)
Hours: Monday-Friday: 9am-5pm ET /
Saturday: 9am-2pm ET.
International Shipping Information