The best place to look for string for your string art pictures is your local craft store. The characteristics to look for in the ideal string are:
1. Strength.
2. The right elasticity.
3. The right thickness.
If your craft store has a cross stitch section then the multi-stranded cotton sold for this is perfect. It comes in lots of bright colours and has the right elasticity and thickness. If they do a value pack with a number of different colours then that is the one I would go for,
You could also look in the knitting wool section. If you can find silver wool it would be good choice for string art. Silver shows up particularly well against a black background.
I was in my local craft store recently and as it is getting close to Christmas they have all sorts of coloured strings for making decorations. It is worth buying a few different types just to experiment with.
What is your favourite stringing material? Please let us know in a comment to this post.