wells heres a nice little device i found
http://www.linuxslate.org/Review_DRSB-88.html#links
but its kinda hard to source them, i cant find any sellers yet
i also recommend giving this a quick read
http://radmeters4u.com/#1a
really it depends on your needs from it! are you looking at something that'll give you a heads up on a dirty bomb a few minutes before everyone else? something that'll tell you that the average background radiation is changing?...if so does it need to be something you have with you at all times?...like some of the novel geiger watches
or something like this
http://www.gammawatch.com/ (there are other alternatives too, especially since japans nuclear troubles...they like they're technology after all ^^)
or even like this
http://www.nukalert.com/
if you looking at something that'll give you good readings over a long time (although im no sure why you'd need it, unless researching?) then you'll probably need one of the larger "block an wand" sorta things
just read about, do some novel searches on google like "smallest geiger in world" or "geiger watch" you'll be surprised where the links take ya!...graet way to find some nice stuff