You can go either way. If you're planning on experimenting with Miracle-Mineral-Supplement long term then I'd recommend making it yourself. However, if you're thinking of trying it out then I'd recommend buying the ready-mixed solutions.
MMS is typically made by mixing a 28% Sodium Chlorite solution along with a citric acid solution. You can buy the raw ingredients: Sodium Chlorite powder & acid, to make it yourself, but.. the only reputable company I know of personally, only sells bulk(1lb and up). Which would make a substantial amount of Miracle-Mineral-Supplement . However, if you're planning on trying out Miracle-Mineral-Supplement , then you can pick-up a ready mixed kits online from ie. eBay under the name: Sodium Chlorite OR Master Mineral Solution.