Hi Paul,
the pistons i fitted were part no. 42H-11636-00-00
I got them through a yamaha dealer.
Total cost including rings etc was about 270GBP
The TR1 part no is 5A8-11636-00-00
You'll notice the part No is the same, it's just a different prefix so 1TA-11636-00-00 pistons will fit as well
Try your yamaha dealer 1st - they may need to back-order from japan which will take about 5weeks for delivery but is likely to be a bit cheaper than cms
http://www.speedsupplies.com/cartdisplay.asp have pistons at 175USD
good luck!
Bob