Imperial exited the market in 2013 and Heirloom has exhausted its supplies of spare parts. Heirloom has identified for most products an alternative button/valve combination.
From the charts above go to the header or the type of cistern (plastic or vitreous china) you have and then identify the button that matches the flush button in your cistern. You can then look to the righthand side and identify the valve that is inside the cistern. On the very right hand side will be details of the Non -Imperial valve required for your toilet.