is very stiff unlike the 'silver bareing' soft solder used for plumbing, then any refrigeration and heating supply will have it. It comes in a premixed form and is a white paste. When heated it turns clear. If this is what you want and all else fails, I'll send you some. I have 3-4 partially used containers. If you get some and it's dried out just add a little water and mix. This flux is 'active' between 800-1600 degrees F.
Here's a link to what I think you might want.