If one has a table saw, it is easy to set the blade at a 45 degree angle and rip the blanks from a board.
While many do not have access to a table saw, they may have a scroll.
To cut a corner blank with a scroll saw, start with a piece that has a 45 degree side, and use it as a guide. You can purchase 1 x 1 x 12 practice sticks, and place them up against the guide as shown in the following photo. Hold the 1 x 1 x 12 against the guide piece and move both into the saw blade at the same time to cut your corner blank.