I do not have to explain to you how important it is to have the right supplies for being able to do what you want to do. And now that I'm trying to sketch and paint out in the world, I needed to find the right travel kit. And I think I have it! I know you want the details.
First, the bag. I've used various containers over the last few months, often one of my African baskets, But while those worked adequately for some situations (say, hanging out at Starbuck's), it wasn't a good option for sketchcrawl sort of walks. (Well, the one I did.) So, I did extensive internet research, and came upon this bag by Tom Bihn, a travel bag company in Seattle. This is the large Cafe Bag, a style of messenger bag that suited my needs perfectly. (I agonized between the medium and the large for some time, then figured that there was only a few inches' difference and I'd rather have a bit too much space than too little.) It comes in all sorts of colors. Mine is actually somewhat darker than the way it shows up in this picture.
Stunning green interior, isn't it? So, you see the big zip pocket on this side, and the main compartment opens on top.