Fillet brazing is often referred to as Braze Welding since the filler is maintained just above the solidus temperature, allowing buildup of the fillet.

Tubing - Just buy low carbon, seamed and welded tube to start. 0.028 to .035 wall. AISI 1010, 1018, 1020 DOM, whatever you can find.

Filler - Any filler that meets the AWS specs. I'd start with RBCuZn-A, -C or -F. I'd get both 3/32" and 1/8" rod.
Flux - Get AWS Type 4 paste and powder fluxes. Paint the joint with paste, coat the filler rod with powder to start, fine tune to your preference later.