There's nowhere that it's readily available for cheap. There are always copies on eBay and places like that but they're usually expensive; it's always possible you'll run into a good deal though. If you get lucky you can find it at a low price from a family member or friend, or from a garage sale or something.
Since you already have an .iso and a Wii, all you have to do is mod the Wii and install a USB loader; you don't need a disc.
A lot (I want to say most) of the people who started playing in the last few years who didn't already happen to have a disc just pirated it and ran it on a homebrewed Wii. When you're playing on your own you want to use the
20XX hack pack, which requires a modded Wii and a modified NTSC 1.02 .iso (assuming it's the correct version). Because of this, if you do get a disc you'll probably end up only using it to bring to tournaments and to rip the .iso from to make the 20XX pack .iso. In fact, since it's about as easy to pirate an .iso as it is to rip your own, you might not even bother ripping it and pirate it anyway instead.