This is the only rectangular one I can find. OUT OF STOCK of course
Fits hole size 65mm x 35mm originally made by Fluidmaster for T.C Finesse w.c
View attachment 60706
Another button on the link is said to be compatible and appears to be in stock, both types would need trimming to length.
Genuine Manufacture Fluidmaster TC Bathrooms Toilet Cistern Push Button. Replacement For Finesse Model. Dual Flush. Chrome Plated. To Fit Hole Up to 70 x 40mm.
www.jslbathrooms.com