I buy sponges like that directly from China. https://www.fasttech.com/p/3367200 (Not affiliated, just in general a satisfied customer)
I agree that they are very useful and user friendly.
The ones I buy are marketed for fingernail care. Thus, all sides are very fine grit and extremely useful for very light sanding and then polishing your surface back to gloss. (I’ve used them on (vintage) plastics, steel, stone, acrylic, ebonite, brass all to good effect)
Last time I bought them when the site had a sale; 68ct a pop. Have so far used up one, broken in another one at work and one at home.
Will get more some time.
The 3m ones are great if you really want to blend lines and surfaces. If you need to keep edges sharp you’re going to need something firmer than these. I’m a fan of gluing zirconia sheets to either hard rubber or corian blocks. The ceramic will last a lot longer than the 3x or 4x, and the harder backing will keep distinct edges and angles distinct.
(favorite sandpapers: Rhynowet for silicon carbide, and Norton “Norzon” for zirc.)
