[link=http://theringlord.com]TRL[/link] is my usual source for square stainless. There are a few other places that offer it as well, but generally you're overcharged like crazy. TRL hurts me on shipping, since they're in Canada, but with large orders, it's worth it.
Actually for learning HP using two ring colours like that is ideal so you can better visualise everything going in the right place. HP 3-1 is often thought of as two separate stacks of rings interconnected. One thing that helps some people is to look at the eyes of one stack. Eyes being where two rings on the same plane overlap. If you consider an open ring, put one end in and up from the bottom, around the first eye, then down from the top through the next eye.
I've already gotten a request to repeat the Celtic Spiral earrings (the ones in the middle.) With a couple different colours. Sapphire and Diamond swarovski hearts. But I think I got myself addicted to another long project before those get done. ;>
Mmm. http://www.cgmaille.com has some of the most awesome tutorials for weaves. Not a massive list, but the ones there are very good, descriptive, and contain fabulous images by Phong. Who I hear is currently working on a book form of his website.
If you don't find a tutorial for a maille weave that you're looking for, feel free to PM me and I can work one up or locate one for you.