Yes, a null modem cable is a cable where the data and control pins are crossed over in order to connect two devices with similar ports. This goes back to the days of mainframes where you normally had a Data Communication Equipment (a DCE, ie: modem) connected to a Data Terminal Equipment (DTE: terminal or printer). When you wanted to connect a terminal directly to a printer or another terminal, then you null out the (non-existant) modem by crossing over the signals.
For the straight through RJ11 cable, I guess that any place offering custom made cables should be able to make you one easily.