Did you try experimental/ultralight homebuilt aircraft supply companies in Europe? Sheet aluminum a staple supply for the aircraft maintenance folk. Even a nearby airfield or serious larger aircraft maint. business will have various thicknesses, and a call to supply could get you a link.

I can already tell the knife will be beautiful. Would not a harder higher silver content solder be more corrosion resistant over time?

Am asking, as I hate seeing older knives which are perfect except for badly deteriorating solder joints. Knives which are used are constantly exposed to stuff which attacks solder far faster than steel, whether blood or salty juice from cooked meat.