The postal stamp is just under 1,5" long and just over an inch wide (3,5x2,5cm).
If you want to find a stamp like this I suggest you google it or check out 'TOGA Printemps Rubber stamps'. TOGA is a French company and I presume they do not sell overseas. I do cut around mine although having a postage stamp punch would be great.
Good luck in finding one! Before I go I have made another one