I've never been one for a lot of manipulatives or curricula. I have ordered a handful of schooly books over the years, but mostly the only thing we buy for our homeschooling are good books, "living books" as some call them (as opposed to text books) and nice art supplies. Hands down our most used supply over the past years has been clay. I ordered one pound of something like this clay from DickBlick art supplies more than three years ago. Its still going strong, and we use it for everything including bike repair, electronics experiments, holding tiny things in place while painting, creating air tight seals for science experiments, to stick things to walls, standing up model "trees" on our seasonal table, shaping letters, for fidgeting during math lessons, and for creating volcanoes, mountains, and water shed relief maps. Oh, its pretty good for sculpting too.
I find this stuff all over our house, but its never been an issue--it never gets overly ground into the rugs, and hasn't ever stained anything. Occasionally I'll find a clump outside, take it in, rinse it off and its good to go. I'd say we have about half a pound left so it may be time to order some more.
I scrolled through our photos and found more than a few pictures of clay in use over the past year: