Most hangers use 1" (2.5 cm) polyester or polypropylene webbing which can be bought for about UKP 10 for 50 metres (Google polyester webbing). make your own by sewing or using figure eight type knots (see professor Hammock on Youtube for methods). Car seat belts would certainly be adequate, but probably more expensive.
Sgt Rock, who is a guru on Hammock forums for lightweight hammock systems, used 1/2" (12mm) Mule tape, which has an unknotted breaking strain of about 1000 lbs, and as he says, he is not dead yet. However, he is a skinny little bugger and others might feel safer with 25mm webbing.