http://www.pgp.com (they have an online store)
You'll need a copy installed on each computer you want to use to read the contents of the encrypted flash drive. (The
Whole Disk Encryption offering is what they'll point you to for encrypting files, disks, flash drives and their ilk.) Both WDE and PGP Desktop are now available as Universal Binaries.
It's not free (pricing is available from the PGP webpage). There are free alternatives (
http://www.gnupg.org), but you may find that that may be more of a headache than it's worth.
FWIW I've been using PGP for a long while now both under Windows (XP) and OS X and have been happy with it.
Intel Mini 1.66 GHz, Mac Pro 2.66 GHz Mac OS X (10.4.8)
Vista RC2 and XP running under Parallels
Message was edited by: Robert DuBose (added online store cmt to PGP URL)