Where do proXies come from?

I have often been wondering how can some sites sell 1000 ProXies a day, how do they get hold of the proXies???

 

mommy proxies?

 

They are regular servers where proxy ports have been left open and/or the correct type of server software has been installed.

 

This is a good answer - there is also the illegitimate proxies spawned from a fleeting romance.

 

I hope there will be a software to identify proxies.