I don't know offhand of an open-source captcha library that's nearly as effective as reCAPTCHA. All the old "wavy lines through slanted text" ones are completely broken with current OCR techniques.
The alternative is nothing. There is no justifiable reason for a CAPTCHA for looking up voting locations. It can be a static HTML page served to anyone in the world.
But there should be accessibility laws to prevent government agencies from blocking access to citizens/taxpayers using these proprietary technologies.