Edit: With the help of comments below, I've changed the question somewhat.

Venus, real color

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 ?

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    $\begingroup$ I'm sorry, you're proposing to launch acid clouds into space, and you expect them to stay clouds? Perhaps economical is the wrong word. $\endgroup$ – GdD May 7 at 15:14
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    $\begingroup$ Venus' greenhouse effect is due to 90 bars of $CO_2$ and has very little to do with sulphuric acid. Furthermore this is more a world building question than space exploration. $\endgroup$ – AtmosphericPrisonEscape May 7 at 15:33
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    $\begingroup$ 2) Because Venus is so bright from space (its Bond albedo is 0.75, 2.5 times that of the Earth), it receives less net solar radiation than does the Earth. Venus is hot because of its thick CO2 atmosphere, not its clouds. If you want to cool Venus you need to get rid of 99% if its atmosphere. Teleport some of Venus' atmosphere to Mars and you'll solve the problem of Mars being too cold! $\endgroup$ – David Hammen May 7 at 21:14
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    $\begingroup$ (3) Science fiction does not belong here. Questions in the science-related corners of the SE network are supposed to have one correct answer. There is no right or wrong in world building. Ask this question there (but don't expect a valid answer). $\endgroup$ – David Hammen May 7 at 21:15
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    $\begingroup$ The clouds are already shadowing Venus. Surface temperatures are what they are despite the clouds reflecting most of the light away. And they'd evaporate in vacuum, and it'd be extremely difficult to get material from that deep in the atmosphere out into orbit, and it'd be virtually impossible to keep it where you want it... $\endgroup$ – Christopher James Huff May 8 at 0:06

Photometry from the Helios and STEREO spacecraft suggests that there is a narrow circumsolar ring of dust associated with Venus's orbit.
The publishers of the article Co-orbital Asteroids as the Source of Venus's Zodiacal Dust Ring found that ~8% of the asteroids in 1:1 resonance with Venus should have survived the age of the solar system, enough to supply that ring of dust.
They predicted the total mass of the ring and that it translates to an asteroid with a radius of ~1 km, which would have a volume of ~4 km$³$.

It would take some time, but with this volume a 1 mm thick, 100 km broad band of dust could be created along the equator of Venus.

Or would it be easier to create a 2000 km diameter dust disc at the L1 lagrangian point ?

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