How big are the sheds? How steep is the pitch and how handy are you? I would make shims from treated deck board. That should last a long time. Custom fit each one. Use a hand plane to adjust thickness or have a friend with a bandsaw cut them for you. If the sheds are large enough to need continuous support, buy some 4x4 or 4x6 beams and use the shims to level the beams and put the sheds on top of those. Hope that helps.
Jim