Try Paul Stuart or J Press in NYC. I would suspect that Press would be a bit cheaper with likely comparable quality. If you can wait a bit their sale will start in a few weeks. I have a couple of PS's grenadines and they're quite nice. Ben Silver also ought to have some.
www.bestofbritishshirts.com has some by Oscar Udeshi grenadine ties that look interesting, though I don't own any. I've bought from the Ben Silver and J Press websites. You might also check www.fourinhand.com (I haven't visited the site recently, so I don't know if Jonathan currently has grenadines or not).