Cotton string bags are fantastic. These are my best bags and I use them most of the time. They have many virtues including:

I bought mine from Organic Ally”

She sells bags with either long or short handles.
The short handles are great for supermarket runs, the long handled bags can be hung off your shoulder for more leisurely shopping – I don’t like them as much as the short.

Bought in bulk i.e. numbers of 5 they are cheap – from around £4.00 a bag inc vat and shipping. You can mix and match bulk orders so you have a combination of short and long handled bags.

Make Your Own?
Or you could be uber-green and knit your own string bag – lovely bare foot person has posted a pattern for cotton yarn and a knitting machine.

Fold Up Bags
The only downside to string bags is that they are full of holes which means that they are see through and they do not fold up up as small as the opaque  nylon fold up bags

I like Onya bags …..BUT they are made from a synthetic fibre RPET
WHAT IS RPET?
Post consumer bottles are collected from recycling plants and spun into yarn. From this yarn comes the fabric that makes Onya Bags.
So, you might think, plastic bad…. but recycling good. Confusing
Well yes – but I have bought them anyway. In fact I have other nylon fold up bags that are not made from bottles. They are very convenient if not quite as ideologically pure as the string bag!

Why do I use them?
Well man-made fabric is not only slightly more waterproof and opaque, it folds up really small. When packed up, these bags are the size of a golf ball, much lighter and come with a clip so they can be attached to your handbag or husband.
I haven’t found a natural fibre bag that is as good at folding, weighs so little or dries as quickly.
They can be carried round at all the times making them an ideal just- in -case bag for impulse purchases.
Moreover because they dry so fast they don’t get mouldy. This makes them perfect for taking on holidays and backpacking trips where laundry facilities are not of the best and space is limited.
Onyabags (UK)