Check out http://newjerseysteelbaron.com/shop/ for 416 among others. I have always used 416 so I dont know about the others.
There are a lot of places to find 416 for sale. Some of my knife dog brothers will be along shortly to tell you where.
I get my 416 from Sheffield Knifemakers Supply. Ann has bars, rods, and some tubes. Good customer service and they carry a lot of other stuff. 416 is designed to be machined and forges well. I only use it for my stainless guards, bolsters, and pins.
Just how hard is 416SS to machine? Does it compare with mild 1020 steel? How about 303SS? I've only worked with 304 and 316SS, and not in knife work, but brackets, supports, pipe, etc in industry. That stuff is tuff!
416SS to me feels like a harder brass with the limited operations I do as a knifemaker. It's slightly sticky feeling when it drills but finishes up very nicely and is much tougher than any of the brass grades/colors. It's not quite a scratch resistant as 304 but a lot easier to work. 316 is a industrial grade like 304 and is likely to be the same difficult product for what we're doing.
I use 416 almost exclusively for guards and butt caps. I really like the stuff. It machines good and polishes up good. I'm not a big fan of brass and haven't used any in years.
I use 303. It's free machining, inexpensive and entirely different than 304 as far ease of working. It's easy to work. But a friend of mine uses 416. I think a difference is that 416 can be heat treated and hardened a bit, maybe less prone to scratching.