You can probably find a Majora's Mask 3DS through your local Craiglist/Kijiji listings, or buy one through Amazon's 3rd party seller. I would personally recommend Amazon because they take your business seriously and will bend over backwards to help you if something goes wrong. I've had a few problematic purchases with 3rd party and Amazon always refunds me 100% and gives me 5 dollars to spend here and there.

If you don't care about having a Japanese version, they're 220

http://www.amazon.com/Nintendo-Legend-Zelda-Majoras-xFF3B-version/dp/B00S66TZ9W/ref=sr_1_4?ie=UTF8&qid=1428195833&sr=8-4&keywords=majoras+mask+3ds

or, for a NA one, 350.

http://www.amazon.com/Nintendo-Legend-Zelda-Majoras-Limited-Black/dp/B00S8IGG4U/ref=sr_1_3?ie=UTF8&qid=1428195833&sr=8-3&keywords=majoras+mask+3ds