Does anybody know where I can attend a professional cake decorating course or workshop?
Are you wanting to take a course close to where you live? If so, which city is close to you?
Are you a beginner or looking to hone your skills?
If you are a beginner I would recommend taking the Wilton courses that are usually offered at local craft stores ie Michaels, Hobby Lobby etc. Just go to Wilton's website and check for the classes nearest you.
If your looking to improve skills it will depend on where you are located or if you are willing to travel.
Here's a similar thread on schools and other options: