51mm is the nominal size for most of those machines. Their actual diameter will probably be somewhere between 51 and 52mm so if you want the tamper to fit properly then youll need to know its exact size or send it to someone who makes tampers to fit baskets and have them make one to fit. There are a couple of Aussie businesses who make tampers but as far as Im aware were the only ones who offer custom sizing (at no extra cost ;)).
If youre only looking for a cheapie then take your baskets with you when you go to a place that sells tampers and try to find one that fits. Anything off the shelf will be 51mm exactly so you probably wont have much choice in the way of size, but at least you wont get one thats too big!