Man everything I can think of for cheap carbs won’t work for you
Save for maybe dextrose powder? That stuff is cheap as fuck especially if you just get brewers dextrose (which is the same stuff supplement stores sell just cheaper)
You can also get brewers maltodextrin which is a bit more expensive
It’s basically just straight carb powder. But if you’re eating 15 fucking cups of oats a day I dunno if it’s gonna be any cheaper for you. Christ that’s a lot.