There are many places where one can purchase a new seat for a BMX bicycle. This includes a specialist bike store as well as online retailers such as Amazon or eBay.
The BMX bikes that you buy at Target and or Walmart are not the same BMX bikes that are used in BMX Racing and BMX Freestyle. Real BMX bikes are lighter, they have diiferent size sprockets among many other things. Feel free to google the National Bicycle League or the American Bicycle League to get more information.
You buy bicycle brake cables, either from a bicycle store, or from an internet source. If you need to be told this you probably need to be told how to install them as well, so go look at www.bicycletutor.com, www.parktool.com or www.sheldonbrown.com
One can buy a custom BMX bike in Las Vegas from a few stores. One can get a BMX custom made from 'Just BMX', 'Bolder BMX', 'Bike World' and 'Vegas Bike Store'.
at the entertainer
Sunday
dk
in toysrus
build it custom yourself
Seat posts aren't proprietary. All you need to do is to figure out the exact diameter (in mm) that your bike needs, and which kind of clamp your saddle require. Then just go and buy one at any bicycle store.
at the warehouse or framers
A bicycle for two is a called a tandem. Simply type " buy tandem bicycle" + your Town or state into your preferred search Engine and see what comes up.
it matters what you are going to use it for