Looking for a sturdy metal rod

Does anyone know where I can find a strong, rigid bar or rod about 6 feet long that has metal loops at the ends for attachment?

The pedant in me says there is no such thing as a non-bendable rod! But practically speaking, you probably won't find one "off the shelf" at your local DIY store. You might want to check out curtain rails, which are quite fashionable here in the UK right now. They come in 20mm, 25mm, and I believe 35mm diameters, with various lengths available. The "finials" that cap off and decorate the ends are screwed on, so they could provide a way to attach things. I've heard people use crutches (!) for this purpose because many have telescopic sections, and tent poles are another good option. You could also get one custom-made by a metalworking shop or a friend who owns a welder—it's an easy job that shouldn't cost much. Alternatively, you could craft a wooden version; I have several different lengths made from broom handles, cut and then drilled to accept heavy-gauge eyelets screwed in.