You can find classrooms for rent in any education oriented venue. Public schools, private schools, community colleges, and universities all have classrooms for rent. These venues are the most flexible when it comes to booking times since they have so many available classrooms. You can book a seminar, lecture, or class even during the school day, in most cases.
Other venues with classrooms for rent include
churches, office buildings, community centers, museums, and art galleries. These venues keep classrooms on site for the occasional student field trip or other educational opportunity. The space you choose should also have access to ample parking if it's a downtown venue. Or at least be close to public transport links or street parking areas.