Unless you are lucky enough to find somebody cutting up a Vega with good floor pans (why cut up a Vega with good floor pans?), you will have to fabricate them. I have fabricated some 2 foot square pieces to replace some rusted areas in my Vega. They looked pretty darn close to the original section. I cut out the rusty section, and used it as a template. It took quite bit of hammering and some shaping on the edge of a table, but everything, including the beads, lined up close enough to weld in with little effort.
Something like this would produce much nicer beads:
https://www.youtube.com/watch?v=-M37lnkkvSwKen