Third Party Air Conditioner Installers

We want an air conditioner in our living room. We have a $200 gift certificate at Best Buy, so we will probably get one there. We can get it home ourselves.

The windows in our living room are unusual. Two horizontally sliding windows above and a 20" x 40" pane of glass below. The air conditioner would be installed in the pane below, like the conditioners in our bedrooms.

We are looking for a third-party installer who is certified and can provide the paperwork our co-op requires and who would be able to install the air conditioner in our window, which would require a customized plexiglass frame in the space around the air conditioner.

Can anyone recommend installers? We know that PC Richards would do this sort of installation for about $150 if we bought the machine there, but they won't install anything purchased elsewhere.

Thanks!