You will find something small enough eventually.
I found the ones I used before I got originals - a packet of blue and pink and white elastics which were perfect in size and colour [only a little too wide] - in the kids department at a department store. Completely random finding. The colours and size you want will probably turn up in a childrens selection rather than anywhere else. There is no brand or anything specific to track down, you really just need to try different stores and sources until you find some which work for you.
Alternatively a little bit of 5mm ribbon tied will work too