OK, since you are on a Mac, you can take advantage of that under the hood Unix goodness.
All *nix systems provide
/dev/random, or possibly
/dev/arandom. These provide random data base on noise generated by device drivers, OS function calls, all sorts of stuff to provide true randomness.
So, what you can do to generate random stuff suitable for password use is a little command line magic. Open the Terminal.app thingie, and at the prompt something like
dd if=/dev/random bs=1 count=100 | openssl sha -sha256
This should give you output like
$ dd if=/dev/random bs=1 count=100 | openssl sha -sha256
100+0 records in
100+0 records out
100 bytes copied, 0.000293454 s, 341 kB/s
and that nice long string can be used as a password.
What is this doing? It is grabbing 100 random bytes of data provided by
/dev/random and instead of outputting to a file or the screen the output is piped to be the input for the
Feel free to use more/less bytes of random data. In fact, if just having a nice long random string works for your purposes you could get the SHA256 hash of files on your system, etc.