I have never discovered this, however i would assume that you could get it from Able Sisters.
No. Sorry :) but there are other cool masks, like hero's mask (link)
just take off the mask
Go to the city and go to Shampoodle's and then tell the person you want a Mii Mask. Yay.
You only need to go to your pockets and put a hat on then take it off and the mask is on your pockets.
Nothing. You just wear it. In Animal crossing Wild World (I'm not sure about the others) there's something I think it's called "pharaoh's mask" (something like that) and if run after awhile wearing it, you'll trip.
That is for if you go to Shampoodles in the city and you got a mii mask. The mask is stored right there if you take it off.
No, your Animal Crossing City Folk character's complexion does not change with your mii mask's complexion.
If by costumes, you mean outfits then of course! You can get them at Sable and Mable's for roughly around 350 bells. Otherwise for example a burglars outfit you can buy the mask and then bu a black and white stripy top. With animal crossing , it's all about combining and being creative! ;)
you can get links outfit from nooks point system for 11000 total points. you can get majora's mask for 6000 and midna's mask for 6500 points
You can give you Mii a mask by going to Harriet's place in Animal Crossing. It's free the first time and will cost $3000 bells after that. Any mii that is saved on the wii is able to be used as a mii mask.
Get a 'Makeover' at Shampoodle's in the city.
Some creepypasta's include NES Godzilla, Ben (Majora's Mask), the lost episode of Foster's Imaginary Friends, and the truth behind Animal Crossing.