Sponsorship comes in a few forms - chiefly free “swag”, prizes and monetary.
If you need regular income and want to keep your meetup free to attend, you're inevitably going to need regular monetary sponsorship. Swag and prizes can be great to attract new members too.
Publishing companies often have user group programs or liaisons that reach out to user groups, providing them with free review copies of books from their catalogs. Here is a list of some of these publishers and their user group program contacts.
Contact (US): Marsee Henon, email@example.com
Contact (Europe): Josette Garcia, firstname.lastname@example.org
Contact: Heather Fox, email@example.com
For review copies: firstname.lastname@example.org
Contact: Niraja Mulye, email@example.com