Agree about the trailer fender - tempting but probably too wide - and narrowing would be sort of an ordeal. Many aftermarket catalogues at a dealer will have a range of chopper fenders. I used a front fender on the rear of my bobber, but if I hadn't been so set on chrome, the typical builder fender might have been a better/easier choice.
Jeff Muller in the club keeps a store of aluminum plate for machining projects that he gets from a variety of sources, including Allen's Scrap. He is willing to part with pieces for very reasonable prices. Not too much tubing, though, I don't think. You have likely seen Princess has a few varieties of alloy for sale, though nothing I ever felt the need for. I find T+S Tubing no problem to deal with - but other people's opinions vary - and I can't say that I ever tried to buy aluminum tubing there, but if it is available, I'm sure they have or can get it. CC Industries (and probably Ali-craft) fab with aluminum and I have bought plate there in the past.