I'm just getting back into the hobby, but I would guess that the lack of 18mm reloads has to do with cost efficiency of Black Powder Single Use motors by comparison. It seems that anything larger than 18mm is much easier to come by in a reloadable format.
And I don't have any experience with them, but even in the short time I've been here on TRF I've learned to avoid Red Arrow Hobbies. Far too many negative posts about that place.
Performance hobbies had them a few year back when I bought mine - might still have some in stock - I'd call and ask. I sold mine locally as I found them a PITA to build and had some of the SU to cover my needs. Seen a set of 3 of them on eBay within the last week.