Do you need a true extension ring, to get more cubic in., or do you need to get an exsiting box flush with the wall, as in an arc shield? If you would like the arc sheild, try this site,
http://www.aifittings.com/whnew120.htm. that is Arlington brand fittings. and the part number is " BE2 " BoxExtender2gang. They have BE1, BE3, BE4, and even BE1R for round boxes. These are used like in cabinet work, where you have space between the box edge and trim edge, creating a fire hazard.
But you will need to go to a supply house for these, ive looked for them at lowes and HD to no avail.
These might not be what you need, just a shot in the dark....