I'm struggling to find a pair of 15 Watt 3TJ type indicators. Does anyone know if another bike uses 15W type or if any of the aftermarket "mini" tyes use 15W?

Couldn't tell you ... why not just add up the wattage and change the flasher unit to suit ?

you can get a full set of indicators for £20 here
http://www.ebay.co.uk/itm/120603675111? ... 1423.l2649
then either change the bulbs for 15W ones about £1.00 each from your local bike shop or change the flasher unit £5-10 to suit
as ec said
and if you change the flasher unit you'll be able to find 23W bulbs a lot easier when they go