Where can I buy or built a subarm base for kenwood l-07d ?

Kenwood made 4 or 5 different bases, each was dedicated to one particular tonearm. So even if you find an original it is likely that it would not match the tonearm you want to use. If you are in the US, a machinist named Colby Lamb, located in Oregon, can make you a perfect blank out of brass or aluminum or delrin. He's made two for me. There was a reproduction made by Vantage Audio in the U.K., too. Colby's work is far superior.