Hi Conrad, the kyosho 2.6mm and 3mm balls have the same O.D. and fit the same rod ends (LA43's). The part number for the 3mm balls is OT32 (OT = optima part), they come up occasionally on ebay (vintage_tamiyashop for one), and are also available here:
http://www.tamiyaparts.co.uk/tamiyaparts/tpx/kyosho.htm, I've ordered from them a couple of times and it's worked so far.
Hope this helps, Richard.