They come in rubber and silicone, by the metre or in pre cut packs of various sizes and grids.
I usually buy by the metre which comes in a very wide width and lasts for ages. I have 1 hallway mat that slips and slides and this stuff is needed, but I have a huge family room rug that is heavy enough to stay put with no help.
By the meter is also the easiest and cheapest way to get your much smaller jar grippers as you can literally cut to the size you want, I like them about 15 X 15cm (or 20 x 20cm) but I do have a couple of longish off cuts that work just as well with one end for the jar and one end for the lid. I have many in a few drawers.
I have also found that if they start to get a bit smooth and don’t grip as well that you can wash them in hot soapy water, dry on the drainer and they get most of their stick/grippy back.