Most gyms offer fitness classes, so check with gyms in your area to see what kind of aerobics classes they have available. You can also try a fitness class finding website or app, such as Classivity or FitReserve. These online tools often have limited coverage, particularly outside of major cities.
Community centers and nonprofit fitness centers, such as the YMCA, also sometimes offer aerobics classes. These are typically a less expensive option than maintaining a membership at a commercial fitness center.
Many gyms and fitness centers offer free introductory passes, which allows you to try out their classes. This is a good way to make sure the gym is a good fit for you and that you like the fitness instructors. Try to take a few different classes to get a feel for the atmosphere and the types of offerings.
To find local gyms, ask friends for recommendations or use a website like WorkoutSpots.com. This site is less focused on classes, but it can provide good leads about gyms and fitness centers in your area. When choosing a gym, also consider things like the hours it is open and where it is located. Choosing one close to where you live or work may make it easier to get to your aerobics classes.