Talk to your local
Gartenbau (garden construction firm) - these usually work with a quarry to source granite. Some quarries sell direct to the public, many only to garden or building firms.
(A bit out of your way, but Küster in Freienbach/ Bäch SZ has a nice selection of stone.)
As for the legal stuff... get thee to the
Bauamt! (Office of building permits, red tape, obfuscation and petty bureaucracy, designed to prevent you from ever changing or improving your property...

)
Don't get me started on my tale of
Baubewilligung (building permission) woe.... One day I may get my
Wintergarten (glass addition thingy) - two years on, I'd be happy with just a garden shed...
Some communities have very restrictive regulations regarding any change to your property, some are very easy going. Talk to the powers that be before you start.
For instance, we had an area that sloped down to an existing boulder wall, and wanted to add extra stone to the wall to level off the slope. We had to track down the original architect from 25 years ago to find the design specs to prove the extra weight could be supported....
Even if you don't need a permit (we didn't for that project, just an engineering study) you might need the agreement of your immediate neighbors. Usual objections would be if your wall would block anyone's view, change water flow, etc. So invite the neighbors round for a very nice apero, and have a chat.
Good luck with your project...