If there are off brands available, get one of those. The cheapest one you can find. I got mine at Target for about $22. As long as it’s 5%, it’ll work the same. There should be dosing instructions in the box. I used the foam type so I’m not as familiar with the liquid if you go for that version of it, but for the foam it was half a cap full. Don’t use more than the amount it says, even if you’re using it on multiple locations.
Also poisonous but to a slightly lesser extent. For cats, even a drop or two will kill them. To my knowledge, minoxidil can still kill dogs, but they are more likely to just get very sick following exposure. If you have dogs or cats, it’s possible to avoid exposure, just be extremely cautious. Wear disposable gloves when applying it, wash your hands very thoroughly and don’t touch your face, don’t let your pets touch your face, and for extra safety flip your pillow over during the day.