Mine came with my boat.
You could buy one new (Hermco/Potter, Armstrong or someone else) or used.
Here is a used one on ebay
web page
Some of our more talented CSC owners have built their own. If you do a search on this web site, you should see some exellent examples.
Also, if you did put a bracket you can then also enclose the transom (which is another very nice thing to have).
Good luck.
Peter
PS Do you have trim tabs with your current setup?