I have not found anywhere besides MAC that carries a commercial brush shampoo, and I felt like the olive oil/baby shampoo method wasn't getting my brushes clean enough, so I decided to search around and make my own brush shampoo.
The ingredients are really simple: water (I used bottled water cause I have it and there's no way I can ever find "distilled"), baby shampoo, leave in conditioner (I used baby type for that too), dish soap, and alcohol.
This concoction really did clean my brushes, with the exception of the foundation and eyeliner ones, which always need a little extra help. It has the same light texture as the MAC cleanser, but the downside is that similarly, it is drying. Naturally that is because of the alcohol so I might try using less next time (when you use the right proportions, this makes a lot!) because I didn't feel comfortable using it on some of my more expensive/softer brushes because I didn't want them to get ruined.