Mining Void Opals with deep core mining is one of the most profitable activities in the game I have found so far.
If you're new to Deep Core or Mother Lode mining in Elite Dangerous, have a look at this handy guide:
You can find Void Opals in ice rings around planets, usually you can find them relatively easily by just jumping to a few systems that have Extraction listed as their primary economy.
Apart from that I have found the following (WIP) list of planets that had Void Opal Hot Spots:
Just have a look around eddb.io/commodity/350 and enter your reference system to find a good spot where to sell them up to over 700.000 CR each, even though their galactic average is low.