depends.....do u want a real copy, or a bogarted one
if u wanna real one, then just about anywhere that sells software should have it. Of course there are sources for the other kind, but we can't go into that here, now can we......
Yes, you need to buy a copy of Windows. System Builder disks (which some people call OEM disks) are satisfactory for Boot Camp Use. newegg.com has them
System Builder disks are licensed for one system only (the ones above anyways but they do have multiple packs also at higher cost), but that is why they are less than half the cost of standard retail installer packs.