I ran into the same exact problem when I replaced the outdoor lights of my house, approximately 2 years ago. I finally had the mounting blocks custom made by a carpenter. He constructed the blocks from cedar, which I painted white. He used the original mounting blocks for templates. He even notched the backs of the blocks to fit flush against the siding, and drilled a hole in each block for the wiring. The worked great.
I tried purchasing the blocks everywhere, with no success. Good luck.