I've made my own starch for years. I use quite a bit since I like my fabric to be real stiff.
I bought quart spray bottles from Walmart. I mix 2 heaping tablespoons of corn starch and 1/2 cup alcohol. I strain the mix into my bottle and finish filling with water. It does settle, so you have to shake it before using. So far I've not had any problems of buildup on my iron. Sometimes I do get the white flakes on dark material, so I just starch the backside and that eliminates the flakes.
I use the 50% alcohol from Walmart. It costs 88 cents. A lot cheaper than Vodka, which has better uses than for ironing! Thru the years I've never had a batch go bad. But without the alcohol, it doesn't take long before it turns into a rotten stink that you can smell all over the house.