@ekaitz_zarraga Thanks for the suggestion - I think we settled on the Bosch ones because they were available in both the US and EU. The small Alwayse ones also have a screw thread on the bottom that makes them tricky to fit in the Ploopy body :(
Suppliers /u/d4baller found:- https://www.boschrexroth.com/en/xc/products/product-groups/linear-motion-technology/ball-transfer-units-and-tolerance-rings/ball-transfer-units/r0530-with-sheet-steel-housing/r0530-1-with-sheet-steel-housing- https://store.livhaven.com/r053010810-bosch-rexroth-ball-transfer-unit- https://l-system.nu/webshop/linear-technology/ball-transfer-units-and-tolerance-rings/ball-transfer-units/type-r0530/ball-transfer-unit-ku-b8-ofk/- https://www.sp-spareparts.com/en/p/r053010810-bosch-rexroth
Spareparts was the one that worked for me in the UK without a VAT number.