I do straight leg raise exercises with a bag of books as a weight. I put my foot through the handle of the bag and lift with the weight of the bag hanging on my ankle. With repetitive lifting, the handle of the bag is slowly fraying.
I am actually very fond of this bag

. I know that sounds weird, but I've carried my books around in it for 20 years. So before I ruin the bag, I want to want to find something else to use for the straight leg raises.
I would guess ankle weights are standard physio or exercise equipment. Does anyone know where I can buy some? Or any other Heath Robinson versions you use?
The total weight of my bag and books is about 15 lbs / 7 Kg.