Hi, welcome 
I don’t know the answer whether there are coupons for Soolantra. (I’m not from the USA, assuming you are. Maybe @misfit108 knows this.) But you probably need a prescription to get it. Once you have a prescription, maybe you can order it from an online pharmacy.
Another option is to use the horse paste version, which is available without prescription and much cheaper. Here is some info about it: Ivermectin for horses/animals
Note that you can get a breakout when starting with Soolantra or the horse paste version, so you may want to time the start with it. Bad first reaction - #31 by col