Many US post office branches offer these.
(Note: Not ALL offer them!  I have the unfortunate
luck that the closest USPO to me that sells them
is about an hour drive away!)  Inquire at your
local post office to see if they do it (some of
the smaller branches, like the one in the little
town I live in, do not...) and if they don't,
then you can call the US postal service 800-
number and find the one closest to you that
does sell them.

The US postal service international money orders
are the least expensive, and (from what I
understand) the easiest to cash.  One to Japan
(it's the pink one) used to cost $3 before the
postal rates went up... I think it only went up
a quarter or something, though, so they're
still super-cheap.  (Costs me a little more in
gas money though :P)

Lady Z