The Toyota fluid would not be made by them but an oil company or the gearbox manufacturer Aisin AW.
Using anything else is at your own risk, it states in the owners manual not to use anything else. This isn't just because they want you to purchase their fluid, all other fluids/oil they give a spec that you can use - Toyota or not.
I have heard of several people that have tried different fluids, including Mobil, and have had poor gearchange or complete gearbox failure as a result.