A friend of mine (not on the forum) coded a little tool that generates English sentences with 80 or 128 bits of entropy.
https://github.com/cjbauer/brainphraseHere's an example of a sentence with 128 bits of entropy:
./genbrainphrase128
the unfolded colorful savings except Mrs. Melody Branch only tames the sorrowful vital golf around Dr. Sheri BradleyThe resulting "sentences" don't make much sense, but I guess that's to be expected. I'm not sure how memorable they are, but at least they're random enough that they should be secure.
It's not a "service." It's open source, so anyone can download it and run it offline.
If you decide to use it, you should probably examine the code to confirm there's no funny business.