I was able to find 1-1/2" in white at Home Depot. They go on without too much fuss. My 2x's were right at 1-1/2" square with slightly rounded shoulders.
In black I could only find 1-1/4". The last stand I made was stained dark so I used those by cutting in a shoulder and trimming off the excess. Don't have a pic of that but it worked.