In that case, I would need an op-amp capable of a 400 mA, 20 V output—and, sadly, my little supply shop doesn't appear to have these. (If it did, I think I could use that for the non-inverting op-amp and be good to go.)

EDIT: I've moved this thread to here.

 

if you are concern with the current capacity of an opamp there are techniques that can solve that problem. I have attached an image of a circuit that can address your concern about the output current of an opamp.