Topre keycaps are different for each row, right? The Verkkokauppa.com spare set doesn't include a key for the zxc-row, which has one more key in ISO layouts next to the shortened left shift. On the other hand, if you get a full 104-key ANSI set, you can substitute a key from the numpad there, assuming it has the right shape.
This is for blank keys, of course.