Edit: With the help of comments below, I've changed the question somewhat.
Credit: NASA, Image processed from Mariner 10 images by R. Nunes http://www.astrosurf.com/nunes
It could seem that the most economical way to cool down Venus would be shadowing it just outside its atmosphere with material from that atmosphere, just because that's "within reach".
I think the best choice for that material would be the particles within the acidic clouds, like the as yet unidentified UV absorber and the possible cloud condensation nuclei like meteoritic dust and volcanic ash.
Like any other material, it would have to get into orbit to stay outside the atmosphere for a long time.
But would that be the most economical way to get the shadowing material or would it be cheaper to get it from elsewhere in the solar system, like for instance (one of the) moons of Mars, Phobos and/or Deimos ?