I got them from a toy/modeĺ shop in my town. The smallest size I could find yet is 0.8mm, but it's amazingly brittle compared to of 1mm. They are very easy to cut with a jigsaw (just use fine-toothed blade, as you was about to cut the same dowel made of brass, copper, etc.)
I know a bunch of adevanced and even some famous modellers, here in Italy, who use carbon dowel to simulate the nailing, instead of brass or copper, to avoid metal blackening in situ, which can stain the wood around the nails when not pre-treated with sealing wax.