I have sourced a few pairs of new original Yamaha TTR250 mirrors. These have the correct left and right hand threads to fit into the existing sockets on your clutch and brake perches.
They will also fit WRFs and WR250R/Xs
It's a personal choice as to whether to to run mirrors but I like to keep an eye on who is behind me!
£26.80 a pair inc UK 2nd Class p&p (1st Class £27.30) - retail is over £42 for the pair without postage.