Looking for window box inserts


I'm making some plant holders, a bit like window sill boxes but ones that you put on the wall, and I have been looking for the inserts. I believe in the UK they are called coco fibre inserts but I can not find out what they are called here anywhere. I tried Gamm Vert but couldn't see anything similar. Do they even exist here in one form or another or would I need to order them from England?

Thanks for your help!