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

Thera are shops who work with acrylic. Some use laser to cut
the form , names, addresses etc. They can glue acrylic plates to
whatever dimension. If you can provide them with a drawing with
exact dimension they can cut for you whatever you want.

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.

Thanks for info
I will need the contact info for Colby