I have several dry bags. Mostly from REI or NRS. They also sell camera specific gear (REI and NRS). I'd recommend starting with NRS. REI is nice as they have stores all over. But I have never been disappointed with anything I've purchased from NRS.

My primary system for the 5DIII with 100-400L mounted is to use place the camera and lens inside a ThinkTank Digital Holster 30. I went with the Thinktank over a few other brands as it took up a little less volume. The downside is that it is a tight fit with the camera. Tight enough that I can't fit the 100-400L into the holster if I mount a quick release plate. It fits without the quick release plate, but just barely. If I am taking my 5DIII with the 24-105, I place that in a Lowepro Toploader 55 AW. Then I place those inside a drybag.