You can buy it from the state board of elections. Most will supply it on a CD. Delaware charged $250 for their list. Can’t remember what MD charged. I bought the lists when I worked for newspapers in the area.
Both parties are supplied lists of registered voters of their own party, and independents, not the other party. So, it does little good, unless you can get some Democrat to turn it over to you.
If you have ever done GOTV calls, you were working from such a list. Lots and lots of old registrations and dead people that are never removed from the lists. With early voting the Democrats can determine ahead of time if those registered voters will vote and vote for them. Republicans don’t do that kind of thing.
I have heard that every single person who ever registered to vote on the reservations still vote, whether they are dead or moved off the reservation long ago.