Careful! Unless you know different, most of the adaptors are for
mice that are designed for PS/2 and USB - they won't convert other mice.
I'm not sure they'll even work on a PS2/USB mouse from a different
manufacturer. And they certainly won't convert PS/2 keyboards.
Which isn't a problem, but be aware just in case anyone was thinking of
buying one just for the adaptor.
There /are/ some real PS/2 to USB converters out there - usually it's that
way round, haven't seen the reverse. These are often supplied standalone
- if you see one with two PS/2 ports, or with other features (RS232,
parallel, etc) then it should work (and would probably work on a RISC OS USB
card too, for the PS2 bits at least). I bought a couple of these from CPC
for work the other week (2xPS2+1xRS232, about 20 quid).