Well yes, true!
It is one of the weaknesses of Johns' Ultraman concept. When Johns showed off Ultraman's origin in the Justice League title we saw him borrow from the old Superman stories aas Kal-il's rocket exits a space-warp along with a large Kryptonite shower that rains down on earth.
Logically though if he needs the stuff to regularly restore his powers the laws of supply and demand would surely take their toll on availability... as with the finer points of he and the earth-3 premise you really can't think on it all too much.
Wonder if synathic Greek K injecting thru his costume work?