do you need bottle jacks....
You can get fairly cheap ...Harbor Freight, Home Depot, O'Reilly's but you have to get them under the structure, preferably resting on a block to and bottom. This makes them a bit tippy however. My preference would be a high lift jack which you can just slip under a joist or plate, and not be under the shed with your hands. Scissors jacks fold down pretty flat, like from a pickup truck...more weight capacity. ... borrow some?
Crushed concrete will work fine then cover it with smaller stone. Then level out your concrete blocks. You may need a shim or two of Pressure Treated 2" X material. place the block so the holes are vertical, not horizontal OR use solid block.
Depending on how the floor structure is built this may work OR not.... no details on the construction have been mentioned yet.
The answer to your question will only be as detailed and specific as the question is detailed and specific. Good questions also include a sketch or a photo that illustrates your issue. (:< D)