I'd start with one of the old gallic acid formulae from the 1870s-1880s. Google for developers for use on dry plates, you should find what you want. What you get by evaporating acorn tea will be of low purity relative to gallic acid, so you'll probably have to use a good bit more of it than those formulae call out.

BTW, you can also extract tannins from black tea, though I don't know if the yield is high enough to be worth paying for the tea (even at only a couple cents a bag) compared to picking up acorns in the park (or your front yard).