3M Safety-Walk, Slip-Resistant, Non-Mineral, Medium Resilient tape, in black or gray. I highly recommend this type of grip tape as it lacks the abrasive minerals (sand/grit) which would otherwise shed off and abrade your screen and housing.
It's typically available by the foot in 1, 2, and 4 inch widths at a quality hardware store; bought mine at a well stocked ACE Hardware. The full UPC (may need to drop the first and last digit) for the gray 2" tape I bought is: 0 51131 59507 1.
Great texture and safe for any surface you may set your tab down on too. Self-adhesive back; just peel and stick (excellent adhesion providing the surface you are applying it to is clean). If you try to buy from 3M you'll have to buy a full roll and it's not cheap ($60+). Occasionally you might be able to find smaller sizes of the same material packaged in little kits as non-marring grips for shoe heels/soles or in strips for tubs/showers (maybe at a Target, Walmart, Kmart, or major grocery store).