um yes there are. you just have to call in the MMA gyms in your area and ask if they offer drop-in. thing is, if you take classes once a week, it'll probably be worth it to just sign up for the month and ditch the gym membership, because you'll be getting more of a workout with an instructor pushing you than pumping iron and have yourself be in control of when to stop.
also if you do drop-ins, it's WAY more expensive. drop-ins vary from $5-$20 depending on the gym rep and what martial art you're going into. if you spend like $80/month, going in once a week, you might as well sign up for the whole month for like $100.
it's just cheaper to sign up long term and ditch the gym membership and workout in the mma place.