I don't know if this link will work but you can buy them cheap..
https://www.alibaba.com/showroom/12v-electromagnetic-brake.html
https://www.alibaba.com/product-detail/DHM8-12-Spring-Apply-Power-Off_60291147577.html?spm=a2706.7843667.0.0.7c1a151dEOwFKF
Gear boxes will have a small brake off the back of the motor (let the gear reduction work for you) and are spring engaged and electrically disengaged when the motor is powered.
Something like this.
https://www.stearnsbrakes.com/products/motor-brakes/servo