Sorry Larry, the only advice I have to to try talking to Arris again and as VulcanRider said speak to the supervisor. They should be selling the batteries as its their product.
The only thing I can immangine would be the problem will be if they are not selling to the general public and you have to go through an authorised resller ( Which Time warner may be )
Also if you take out the battery and see what brand,make,model it is you may be able to buy it online or from a batery shop.
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