I purchased mine (GMEPP Platinum 8y/100k/$0) last weekend from Knapp. They had the best price of the dealers I reached out to. You can get an instant quote and purchase it online. Best of it, is that it's transparent and website allows you to see pricing for all protection levels, duration, miles and deductibles.
https://www.knappvehicleservicecontracts.com/
Also, as others have mentioned, extended warranty starts on the day/mileage you purchase it and only stars to cover after your original warranty expires.