What kind of ladder on slope?
With a solid surface like that, that is sloped a little bit, a few sizable scraps of plywood or short piece(s) of 2X8 under one side is all that would require. I know it sounds sketchy if you haven't tried it, but with a solid sidewalk under it, it will be surprisingly stable. On job sites, I have used rocks, brick, 2X4, anything to get the leg even with the other. Of course discretion is used how these are placed.

If this is something you want to keep handy, a piece of 3/4" plywood, say 2 ft X 1 ft, which should be enough to comfortably set a ladder on, and fasten wood blocks on the underside, one at the low edge and another in the center that are 1ft long as well to level the plywood  "base" for the ladder to set on. You can hang it on the garage wall until needed again.