Dp found it amongst his nerdy blogs...! a fluttery snowstorm to hang in a window, made of plain white circular stickers.
here
It says use dental floss, but we used fine sewing thread thinking it would be a bit safer if a child pulled it down (as it's less strong and would break if pulled tight).
Also, we made ours hang at random lengths to roughly about 2/3 of the way down the window, to keep them out of reach of visiting toddlers and the cat - and have filled up the bottom of the window with paper snowflakes. Looks really great.
I got the stickers from woolworths, £1.50 for two generous packs.