Hardware store, farm supply store. Sometimes they have it in stock (usually just in galvanized or maybe 1-2 colors), sometimes it's a special order item (and there is sometimes a minimum size for special-orders). It is usually 2-3' working width and comes stock in 8' and 12' lengths (tho if you're special ordering a decent amount you may be able to get custom lengths).
Is there something besides shingles you can put over a 1/2" wooden roof to make it water proof that doesn't involve nails sticking out?
I use a shower curtain stapled over the plywood on my tractor, does that count?
(I actually do. It has survived a total of 8 months of use out in the sun, plus two winters of storage, and is still going strong, but I have no idea how long it will remain so).
You could use tarpaper, but there is a limit to how waterproof it is and for how long it'll stay that way with no shingles on it, especially if it's a flattish roof.
You could just prime and paint the cr*p out of the plywood, that'd last a few years before the paint needed replacing... you might get 5+ yrs out of the plywood before the plywood needed replacing. I'd caulk real well over whatever screws/nails hold the roof on (where they penetrate the plywood)
Most satisfying solution might be to install battens every 2' (or maybe further apart, since you have the plywood there - depends partly on your site's windyness) and put on metal or corrugated plastic roofing. (e.t.a. - by preference I'd put the battens on the *inside*, not on top of the plywood, so's not to create places where water getting under the roofing could pool and start rot)
If you use proper, gasketed roofing screws, of an appropriate length to hold the roof panels on securely but not penetrate thru the battens (and of course you have to locate your screws in the right place so they DO bury themselves inthe battens
), it will be quite waterproof and pretty long-lasting. I'd paint the plywood underneath it for water-resistance, and leave the lower end open at the corrugations so that any water/condensate getting under there can drain/evaporate on out.
Good luck, have fun,
Pat