Not sure where you could find one.
Just thought I would throw my 2c in though. If you source another HWS for cheap, you will have to have it installed, along with changing any piping around to suit - not to mention that if you don't have a Tempering Valve currently installed, then the plumber has to put one in by law.
You might save yourself some money sourcing the same model as the one that has broken, this just means that the pipework can stay as is and this means no tempering valve needs to be installed (due to pipework not being modified). Maybe see what the difference in price is between a second hand + plumber modifying the pipework etc and a new one that only needs to be swapped into place.
Just food for thought.