In the UK we have a shop called Hobbycraft (you can order online yaiy) & if you look in its thread section there are some tinsel threads which appear to have the same texture & thickness as Pony tinsel. There's another tinsel thread here & I posted a picture of Xmas tree, who has green Hobbycraft tinsel thread through her mane & tail. The thread comes in a variety of colours including iridescent.
I'm sure other countries have their own craft shops with tinsel threads.