Home Depot, bulk sections. Where they likely have a giant rotating rack of wire reels, not the actual speaker wire section.

However, if you can wait, most of the other choices mentioned above are much easier to work with. The Home Depot stuff is super stiff and is so hard and brittle at the ends, you might want to wear gloves when your working with it. Fine for runs you won't be playing with again for a long time.

With great power comes Awesome irresponsibility.