Just about any
inexpensive audio cassette recorder. 
 Funny how those have all but disappeared too,
though - they were
 everywhere at one point in time, but I've only seen a couple out here in
 the last four years of rescuing interesting things from junk stores (one
 totally snafu, but the other was quite a nice Wollensak one, with box,
 case, manual, mikes etc.) 
 
    Easy to find in Brazil, even the special models made specifically for a
said computer (like DR1000 from CCE, or the special datacorder from
Gradiente and sharp for the MSX computer, etc). A very common k7 recorder
used in Brazil was the RQ-2222 (and somewhat the RQ2211) from
national/matsushita/panasonic