No, not necessary. The Mounting brackets are of course supplied with the mobos, but in a case of third party assembly, it is not necessary due to customization and limitations of customized cabinets or mods it is upto the third party to decide what is needed in the cabinet.
Newegg will send you the bracket if you buy it from them.
You bought the PC from iBuypower actually, Newegg was just a retailer inbetween you and ibuypower.
Although it'd be easier for you to just walk down to the nearest computer hardware store around your place and pick up a compatible HSF bracket to go with the Arctic cooling thing.
I guess anyone of these is compatible with your mobo
Hope it helps
From what I gathered of your description of the problem, I think the Back Support plate (Backside of the Mobo, a steele plate is screwed to the Front side bracket )
I guessed both are missing, the back support plate and the oval-ish plastic perimeter sort of wall around the processor socket in which we mount the standard HSF (Heat Sink Fan) right?
Well, anyways whatever is missing in that set you can find in the above given links, as a matter of fact if something else is missing, you'll be able to find it on that site too, small spares, fixtures, mounts, etc etc
Yah, it is frustrating, but I guess, I've been in this line too long, well that's the only way the computer companies can keep functioning. Like they say in our part of the world " A car mechanic always leaves one loose screw in the car , so that you return to him "