Spacetrader Beads is a wholesale supplier of beads, findings and other craft products for jewellery making including beads made from glass, gemstone, crystal, shell, wood, metal, sterling silver, resin and more.
Waxed cotton is a very common product for beading it comes in many colours it is very strong and is supplied in various gauges such as 1mm or 2mm- so it can be used in small hole beads
(if you to use 1mm leather it would probably break but wax cotton will hold )