I have been buying my bands from Amazon for years now. They come in different sizes but I find the 1/4" ones work best for us. Usually 2 or 3 times around the topknot are plenty to hold it nicely. They are the exact bands my groomer uses.
Amazon.com : Latex Free Dog Grooming Bands by...
Usually I do two bands on the topknot. One if front and one behind the first one. Then you place your bow between the two sections and it makes it very secure. Bows can be choking hazards so never leave your dog unattended when wearing a bow unless you are sure you secured it properly.