It sounds like your problems in getting this done are more political than technical....
- No, you can't have a test set
- No, don't run your car for power
- No, don't use your cell phone
However, all is not lost. Most DSL modems are powered by a wall-wart type of transformer that produces a nominal 12-volts. A small 7AH SLA battery should be sufficient to power one for several hours. For that matter so would eight "D" cells in series. Visit your local Radio Shack (or Fry's, at least an order of magnitude better stock) for the wire, connectors, etc., or sacrifice a wall-wart for the wire and plug.
If you have 48 volts DC at the remote site from span power, you can use a small converter to step it down to 12 volts to run a modem. Something like a PSD-45C12 from mouser.com is around $18.
Converting to 120VAC just for the wall-wart plug of a modem that steps it back down to 12 volts DC won't be very efficient.
Read the specs on your modem's wall-wart to be sure, most of them are 12VDC.
or if you have -48VDC at site, cheap inverters are available to change your -48VDC to 12VDC to power modem, we do it at a lot of our remotes
All we have are the pairs from the LPU about 10,000 feet away.-48VDC would be nice.