I carry several pieces of 2x12 treated wood that I cut into 12" pieces (so, pieces are 12x12x2).
Plenty big enough for the jack pads, with a little extra room in case I don't get it in exact right place. Costs very little, and can also be used for leveling blocks when needed.
Used to use plastic pads like the one mentioned above. Problem I had with them is their smaller size means you have very little error margin in terms of where you position the pad. Meaning, if you don't sit it EXACTLY where jack pad will come down, the pad will not be entirely on it, making it not quite as solid/stable. With 12x12 blocks, I have a bit more error margin to work with.