Cassia buds are not as commonly used as other parts of the cassia tree but they are still useful for providing a novel twist on the cinnamon flavor. However, the fact that cassia buds are not a well-known spice means that you may have a hard time hunting them down. If you are able to find cassia buds somewhere, they will probably be expensive. Consider one of the following easier-to-find (and probably more affordable) cassia buds substitutes below.
Your best bet: Cassia bark (cinnamon sticks)
Most of the cinnamon used in the US is actually the rolled inner bark from the cassia tree, which is also known as false cinnamon or bastard cinnamon. It is characterized by its reddish color and extremely hard texture. The flavor is another important characteristic as it is almost the same as that of cassia buds and is a result of the fact that they both come from the same tree. While the bark may lack the light floral aspect of the buds, it will be indistinguishable in many applications. Left whole, it is sold as cinnamon sticks but it is also ground to make cinnamon powder. It is so hard that you might as give up on trying to grind it at home. It is far too hard for conventional spice grinders and may even be too hard for all but the most powerful blenders. You may be able to grate cassia bark with some graters, but even that method may not work; however, it is possible to break it with the back of a knife or a nutcracker. You can replace whole cassia buds with pieces of cassia bark and ground cassia buds with regular ground cinnamon.
Use a 1-inch piece of cassia cinnamon stick to replace each cassia bud; use ground cinnamon as a 1:1 substitute for ground cassia buds.
A decent second choice: Ceylon cinnamon bark
The Ceylon cinnamon verum tree is also called true cinnamon, which makes it different from cassia. As with the cassia bark, you will be using the bark of the Ceylon cinnamon tree as your cassia bud alternative. Ceylon cinnamon bark is almost paper thin and is much more fragile than cassia cinnamon bark. The benefit of this is that it grinds to powder easily in a home spice grinder or blender. Use that powder as a 1:1 substitute for ground cassia buds. If you want to replace whole cassia buds, simply break the bark into 1-inch pieces and use each piece in place of a cassia bud.
In a pinch: Cinnamon extract
You can make cinnamon extract with either cassia cinnamon or Ceylon cinnamon. Since you are trying to replace cassia buds and not Ceylon cinnamon buds, you should try to find cassia extract, or make your own. Cinnamon extract is made by soaking the bark of the cassia cinnamon or Ceylon cinnamon bark in alcohol. Your best options are vodka or Everclear. The resulting tinctures can be used in many of the applications that might otherwise call for cassia buds. Note that in a dish that calls for ras el hanout or Chinese five powder, you will have to add it separately from the powdered ingredients.
Other alternatives
Cassia leaves are used in cuisines throughout Southern Asia, including dishes from Malaysia and Thailand. In India and Nepal, they are called tej patta and are added rice dishes as well as to curries and soups.