Sectors aren't 'found' - they exist. There are no hidden sectors - what you see is what there is.
As for hidden system, as Xakic says you just mindlessly trawl empty sectors hoping that they put a system in that area, and that you find it. Mindnumbing work, but with a good payout at the end if you succeed.
As for buying planets/systems sure - its been done many many times in the past. But expect to need billions in assets and rare tech etc before you can really think of owning a planet. An asteroid or possibly a moon, maybe, which are 'merely' in the hundreds of millions in cost.