Tony the structural foam niches are available from several manufacturers. The problem is that they aren't really interested in small orders and neither are the distributors who maintain stocks of these on the shelf. They want to sell to paint and wall paper stores, home centers and places like that. These are also advertised in magazines like The Family Handyman. A search on the internet is another place to start.
They are made to fit into the 14-1/2" space between the studs in a normal wall.