There are various commercial brands that sell minoxidil (i.e. Rogaine, Kirkland...), generally in 2% or 5% preparations, using ppg as disolvent. I strongly advise, and it´s the most preferred choice, to get onto 5% minoxidil even since starting, as it have higher chance to achieve regrowth without much difference from the 2% solution.
You can also buy generic 5% minoxidil from your local pharmacy. A dermatologist can prepare the receipt for you.
Also there are different minoxidil brews sold by Dr. Lee or lipoxidil that have another goodies included in formulation, and variants. There is minoxidil with azelaic acid, niacin, adenosine... etc, and Dr. Lee also prepares a higher concentrated minoxidil, called Xandrox 15%. Also there´s a non ppg based minoxidil, very popular, that dries quickly and doesn´t have the inconveniences of alcohol based solutions for the scalp.
I don´t recommend starting with one of those, becouse they´re more expensive and you have to always keep in mind that hair loss is a long term battle, so you have to keep some ammo for the future. Start adding regular 5% rogaine to your big 3 routine and then, when it will lose effectiveness with time, add some improved form of topical.
Personally I have used for two years a generic form of 5% minoxidil, prepared in a pharmacy, and lately (4 months ago) I´ve changed to Kirkland minoxidil, due to it´s lower price and because I can have a 4 bottle pack, without having to go every month to my pharmacy for a bottle of minoxidil.