interior door question

I have an interior doorway between my living room and dining room that was cut when the previous owner converted the garage to a dining room. The doorway is an odd size - 90" high by 35" wide. It has cheap molding around it. I would like to have a french door with a fixed glass transom at the top. I went to a well-known chain store that offers installation and tried to explain what I wanted, but they couldn't understand it. (Typical for this chain...) So, should I buy the door I want (in the size closest to the opening) and then hire - what? a carpenter? - to install it and create the transom? I've never had any remodeling done so this is all new to me.