Build the Domstreuten themselves, what material and where?

I want to weld in dome struts and also a few other small stiffenings (Polo 86c). What material should I use for this? It’s about dome struts in front and back, crossbar struts, struts in front between the Stabi photos and from there back to the crossbar photos. So should everything go with the same material or? Where can I get this from the normal locksmith? Greeting Dominik