Weirdly, cassettes aren't as easy to find as they once were. I usually have to mail-order to be sure of availability. The only chain I know of is Radio Shack, and they only stock their house brand. I did see a large package of cassettes at Best Buy recently, but they were in one big package of 100 or something (they do have a standard cassette dictation recorder on the shelf, though.)
If I was going to bestow a recording device on someone who needed it to record dictation, I might consider a format that can be easily downloaded to most computers and written to a CD (in case a copy needs to be made for someone without cassette playback capability...lots of people...also a CD may be made without the need for a soundcard.)
I'm a great fan of cassettes; I had a Walkman Pro back in the day and made some excellent recordings with it. It seems sad to finally see the cassette begin to go away.
Here's a little Marantz job I saw the other day: http://www.easternelectronics.co.uk/pmd660.htm