A.... http://en.wikipedia.org/wiki/Vapor_barrier

B..... A building supply house. Lowes, Home Depot, etc.

What are you trying to accomplish with a vapor barrier?

Vapor barrier are used on the high moisture side of walls to prevent moisture from moving into the wall.  With a LQ trailer, the high moisture area is the inside of the trailer. You coulld just place a layer visqueen over the wall and achieve a vapor barrier. But if you want to add some insulating value at the same time consider some of the Reflective Foils that are now available. Google Reflective Foil.  These products can be a vapor barrier as well as reflective heat back into the trailer.

As painted horse related, there are several foil faced insulations that provide a vapor barrier. The ridged foam and roll types both furnish a barrier. The rolled bubble types also provide a vapor barrier. Some wood products that are surface sealed with a varnish will also suffice.

Whichever type you choose, the seams, cuts and tears have to be sealed for the barrier to be effective. The easiest way to seal is to apply a metallic tape over the seams and any small openings.

The sheet plastic is installed over the wall, on a non foil faced insulation, after the walls are constructed and before any interior materials are installed. It is cut and stapled into place, and again any seams or tears are sealed with a tape.

Gard