Most condoms have a standard width of 52 mm – incidentally, the width must always be given on the condom packaging.
You can find standard-sized condoms almost everywhere: In pharmacies and drugstores, for example. A few supermarkets and filling stations also sell them. Vending machines in restaurants or toilets, for example, generally have more than one kind.
By the way: One standard-sized condom is better than none – even if you actually needed another size.