Criggie:nate: ...we sourced some PoE adapters (such as this) from China. Basically plug in Ethernet on one side, and it has a jack on the other, and a small power adapter.
Be careful as I'm sure the limit of data (before it starts to degrade) over Cat5 is 100m.
Those are injectors - some obey POE standards, some don't. The problem is a cablemodem is 240V not 12V.
The cable modems are 12V DC ~1A and run off a plug pack.