Hi there,
Cannot help you on sourcing off the shelf parts - BUT - I would share with you that I fitted some to my 1998 Defender and found out that it made (for me) the driving position impossible.
I found that the sun visor was eye level so I was stooping all the time, also, to look out of the side window I had to look down to see out.
I'm only 5'10, so not particularly tall or anything like that - suggest that you try driving on a cushion of the same height as the risers and see how it is for you.
Good luck with your project.