There's also another trick you can use in a pinch, mixing a specific ratio of baking soda (sodium bicarbonate) and lemon juice (citric acid) to get a crude solution of sodium citrate to use in cheese sauces.
The problem is it'll give a slight lemon taste to whatever you're cooking, and it'll be less effective since this reaction forms a lot more monosodium citrate than trisodium citrate, which is a weaker emulsifier. It can work in a pinch, but just buy the pure stuff, careful to make sure it's food grade and not bulk chemicals from China.