Doepfer,  a German company with interesting analog synthesizer-related products 
http://www.doepfer.com
 

Jameco,  a friendly supplier of parts and tools that may be helpful for MIDIJACK installation such as digital multimeters (DMMs),  wire cutters,  solder,  and more
http://www.jameco.com
 

Kenton Electronics,  a long-time British maker of interfaces for analog synthesizers 
http://www.kenton.co.uk