How to order XER10

i have 2gbs internet fiber over ethernet but have xb7t modem so its useless how do i get the XER10. Every time i go into the store they say  don't have it and tell me their is nothing they can do and to call support. but when i call support they tell me i have to go to the store to get one. so im paying for internet speeds i cant even use because xfinity wants to play dumb and send you in circles for a modem that probably doesn't exist