I think you're on the right track thinking of
EMT (which is usually coated mild steel, not aluminum.) This
site may help. What thickness do you desire the rings? Anything larger than 1" requires special, expensive, machinery. Would a 12'-8 7/8" diameter work instead of the 14' (as that's 4 x 10' lengths)?
I would see if anyone knew an electrician, or call an electrical company and job it out to them. They can probably get better prices on materials than you can, and will certainly do a better job, as conduit bending is a craft requiring many years of practice to perfect. I'm guessing your largest ring would cost you $20-50, depending on the stock size and whether or not they had it leftover from a previous job. Keep the screws on the connectors to the inside of the circle, cover them with knots and they won't even be noticed.
The above is for decorative, non-weight supporting rings only. If you want structural (and expensive) contact
Tomcat or another truss manufacturer, who would be glad to build whatever you like from 50mm aluminum alloy stock.
Arrow Dynamics bent some 12" pipe stock for a certain project here in Las Vegas, and you don't want to know what that cost! (I didn't know they went bankrupt, but it wasn't due to the Las Vegas job.)
Edit: Just read your profile. It appears you're a teacher, so your first call should be to Facilities. Either there's an electrician already on campus, or they have a preferred electrical contractor who would be willing to do you a "favor," for a six-pack of appropriate non-alcoholic beverage. I don't generally offer this advice, but: "go hang out around a construction site." Outside the fence, of course.