OK I think i've found it, but the minimum order is 200....
https://www.alibaba.com/product-detail/Kool-Mu-Toroids-77254A7-77438A7-77089A7_60794137677.html
Alternatively I could order whatever this is for £5 off ebay, which looks like it has two of what I want on, and then remove them & solder...