I use BeadCord which is Griffin brand, it is 100% silk and comes in a large variety of colours. I have used No.3 which is .50mm as I was using it with pearls, but I also used it to re-string ds amber teething necklace. What sort of beads are you wanting to string? Its a bit hard without knowing what size/weight of beads.. I think they go up to a No. 16 for heavier beads. There are lots of options for stringing, just really depends what sort of look you are after! More than happy to give you some options / websites.