If your toilet is the one installed in the original build and needs replacing, here are some parts that have been used by others.
If you just need to replace the push button system, then others have used the Twyford Optima 49 Dual Flush Toilet Cistern Push Button Assembly (CF1003CP) Siamp.
Finally you can buy a whole new flush system by searching for the Twyford Siamp Optima 49 WC Toilet Dual Flush Valve
If it is just the seal that has gone, you can also buy those on their own.