You can always "home brew" an extension cable. To be done right there are 4 wires in the cable, and though some cameras are actually happy with standard telephone cable, that is not the proper way (But it makes a good try)
You can slice and dice the cables safely so long as there is no power,, There may only be 3 wires
Wires are POWER (usually 12vdc but to be honest it don't matter) Ground (Usually black by the way) Shield and Signal, IF, that is, they did it right and used shielded wire.. .If they did then use standard 2-wire strand for the power and ground, and use standard coax (RG-59 or RG-6) for the signal and shield extnsions, You will need to do some soldering on both ends.
Second, most manafacturers of cameras do indeed sell extension cable that matches the custom plugs and sockets they use. In fact,, I do not know of any that does not do this
Now, if your camera just has 3 or 4 wires (No shield) standard telephone cable and fittings work.