Contact anyone with a voter file (google "voter file purchase")

Trivially: http://nationbuilder.com/voterfile

etc

As for "For free", states are generally required by law to give it to you if you ask. Some charge fees. Only two have crazy fees (5k and 30k) has a crazy fee (though if you challenge them, ...etc)

https://trello.com/b/IlZkwYc0/national-voter-file-states-pip...

There is a github project i'm aware of to put together all of the data: https://github.com/national-voter-file/national-voter-file

FYI, be careful what you do with it. Some of this sort of data--like federal contribution data from the FEC--cannot be legally used for commercial purposes[0]. They do actually enforce it, at least occasionally.

[0] https://transition.fec.gov/pages/brochures/saleuse.shtml#anc... (publication is from 1992, but still accurate per disclaimer at the top)