I've looked everywhere, but I could just be missing it. Has anyone found any gardening items in-game? For example, a sprinkler? Watering all my plants is a pain.

Darn it. I just saw the sprinkler in the catalog and it was in an odd place. I believe it was in the buy catalog, not build. Did you try misc>misc> or deco>misc?
If someone knows..please pipe up and help us out here. :p

Ok, for some weird, seemingly random reason, it is under Appliances>Miscellaneous Appliances.

Whatever. It's also apparently the only garden type item, no ladybug houses, no greenhouse building materials, nothing... how irritating. At least I have the sprinkler now, though.

For anyone else looking for a solution, it's also under "room" "outdoors" "activities." Click the flying football!

If you mean gardening tools, then yes, there aren't much available, except the sprinklers.

If you mean items to plant, you need to buy some produce that are available at the grocery, or harvest some in various lots, or at the science center I think.

There are also gardening skill requirements in order to plant certain type of plants. For example, you need level 7 gardening skills in order to plant peppers.

Hope this helps!