Your local chandler will be able to get one in for you, or any of the online guys.
Full specs:
Scanstrut`s Stainless PowerTower® is created using a highly polished stainless steel, this makes a great looking installation on any boat.
As with all the PowerTower range it is designed to elevate your antenna from the cabin roof, fly bridge or radar arch, to ensure the radar has a clear view of the horizon and maximum performance can be achieved.
Watertight Installation
Pre-fitted IP65 gland to seal cable entry into stem and a watertight base seal included so no need for messy sealant
Pre-drilled mounting holes, the antenna fastens to PowerTower top plate using own mounting bolts
Only 150mm (6") square mounting footprint which makes it ideal for installation on narrow arches or RIB A-frames
Mirror finish marine grade 316 stainless steel
(Pic shows a Furuno, but this is fits anything other than a Raymarine)