I've struggled with this issue as well because I live in a moist climate and instant espresso powder cakes up on me between uses.
Generally recipes which use instant espresso power are trying to add a kick of bitter and roasted flavors to deepen chocolate flavors and make them richer. You can try substituting instant coffee 1:1, but the flavor of the baked good will certainly suffer a loss of richness. Increasing the amount of instant coffee, however, would likely add more bitter or acidic flavors -- and caffeine -- to the cookie or cake than you want. Also, instant coffee takes longer to dissolve, so it needs to be added straight to a liquid.
There's no real way to substitute liquid coffee in most recipes, such as chocolate cookie or brownie recipes, because the recipe has no water in it in the first place. In the rare recipes where there is water, you could substitute strong-brewed coffee for an equal quantity of water+espresso powder, and this would likely work fairly well.
So, summing up: there's no really good substitute for instant espresso powder in recipes which call for it that I've found with some significant experimentation. Hopefully someone else has had better ideas than me.