Seen these in store at all three local Tesco branches in West London over last few weeks - Hayes, Yeading and Osterley.
Three packs - all sizes available S, M & L and packs of just white, or white, black and grey or black and red.
On some days the stock of some sizes has been low but they usually get replenished. They are usually in large white cages and stacked full on the ground (rather than on a shelf) and near store entrances or in Osterley up the escalators in the Extra clothing section. Given they are in three stores around here I expect they'll be in other in-store locations too.
Around half the price I usually see them for as normally around £30
Also don't forget you can double up with your clubcard vouchers making these £8 for 3 or £2.67 each - thanks to Willber00