Voter registration databases are available on:
Lexis - about 30 states (FINDER;xxVOTE for a particular state; FINDER;USVOTE for all the states at once);
Accurint - about 20 states the voter registration is listed under the "Licenses" tab);
Westlaw - about 20 states (VOTERS-xx for a single state; VOTERS-ALL for all the states at once).
If you find another way to get voter registration information, please let me know!