I don't know whether the mounting brackets are available separately, but I have seen at least two different types and I don't know if they are interchangeable.
Do you have the early style bracket as shown at the top of the following picture?
I didn't use my original bracket as it was not compatible with my handlebars. I ended up making a simple metal bracket that just clamped into the slot in the main handlebar clamp on the front of the gooseneck.
Here are the details of the bracket that I made:
I see no reason why this basic bracket couldn't be altered to cater for different attachment points on the bike.
And this is what it looks like when fitted:
Alan