To the best of my knowledge, the only cassettes used in voice recorders are the 'MC' (micro-cassette) type. (That's certainly what my Olympus model uses). They are available in certain High Street stores but they can be hard to find (and, in some cases, quite expensive). However, they're easy to find cheaply online:
hello chris
thanks for nice info
you are a very helpful person
i have been to the site
tried to order
would not take post code
i will give them a ring to morrow
if i win the euro mils
i promise i will not forget you
so thanks a lot once again