The separation between the brackets is determined by the needs of each installation. The material to which the brackets are attached will impact the strength of the attachment, and will vary based on the material. Also, other factors such as 'wind load' of the antenna used, wind speed zone projections for the specific location where the antenna is mounted, potential ice loading conditions of the installation location, and height above the roof line will all impact the needed spacing between the brackets. For these reasons, it is difficult to provide a specific recommendation for spacing between the brackets. However, the greater the spacing between the brackets, the more stable the installation will be.
The antenna installation accessories and mounts can be found here.