Bit confused that you call FTT a .243 pellet because afaik they are true .25
Rhino green, are the only .243 pellet I know of, to the best of my knowledge White Rhino, Marksman, Pylarm are all .25
JSB .25 pellets are officially imported by AA & Daystate but only in the heavy 33gn version for FAC.
AA .25 diabolo are fairly widely available in the shops down here so it depends if the shops around you think there's a market, they won't stock what doesn't sell.
JSB kings 24gn are available on the bay of e,
I've got all the above, PM me your addy & I'll send you a few to try for size before you buy a full tin that's no good.
I do know a thin skirted .25, it's the old Norica Apache but they no longer list it, I can send you a few & if they work it might be worth searching for NOS.
A pellet sizer might be your best bet.