2.5" casings aren't available since a laptop hdd wouldn't fint inside it (they're 3.5"). if u want yet smaller ones, u can get branded portable hdds (not the laptop hdd in case type). they'll be much more expensive.
@choto_cheeta : yes, thats exactly what i was referring to. i got a 80gb hdd for 2.4k and the external case (Zippys) for rs200. total rs 2.6k