if you know the brand, google "[brand] remote replacement"
most likely it is some generic remote controller. you'll probably find instructions for setting it up in the same place you buy it, if it's the same brand, or instructions from someone talking about it if it's a generic substitute. but my guess is it'll have four dip switches in the battery compartment that you'll need to match to the same configuration in the receiver. or just bruteforce it. there's only 16 possibilities if it's set up like that