Hi,
You could always try contacting [http://www.mongoose.com/usa/contacts/|Mongoose] or depending on your location, one of their [http://www.mongoose.com/usa/international-distributors|international distributors] to find out if the part is available and where you can get it from.
Here’s a link to a [http://cdn.mongoose.com/media/wysiwyg/M14-BMX-Owners-Manual_2.pdf|manual] for a Mongoose bike.
On p.15 you’ll find a description of the bike parts and on p.35-40 it details how to adjust the brakes.
Hopefully this may be of some help.