I don't work for this company but I sing it's praises:
POWER PET DOORS ON SALE HERE! Dog Doors Cat Doors, Pet Operated
They do all types of pet doors, through screens, wood doors, siding. We have their electronic model and it simplifies our life and keeps the heat and cold in. Our dogs figured it out in about 2 tries. Customer service is unbeatable. They even replaced a collar that another dog chewed up at doggy day care--no charge. They recently replaced our entire door for only the cost of shipping. It had been installed 3 years ago. Very friendly on the phone and they don't mind questions. Shipping is reasonable. Love them!