Amazon has 20 pages of 48 volt DC to 12 volt DC converters. Most will not have the type of connector your are looking for. Pick a device that is suitable for your current (power) range, voltage requirements and price range. However, not sure how well regulated these devices are but for running a DC LED lamp voltage regulation is not of great importance.
When working with these types of devices cut the wires, strip, tin and use a wire nut to complete the connection. For a better connection solder the wires together, but in the long run that will make it more difficult to remove the connection. However, it is much better for outdoor use.
If you are experiencing a voltage drop in the Ethernet cable because of it length, then tie 4 wires together to make up one pair of conductors that consists of 4 wires. A good color combination is tie the blue/green (4 wires) for the negative side and the orange/brown (4 wires) for the positive side. Strip, twist together and solder together. This will decrease the overall run resistance and I squared R losses.
Many POE switches will only provide power when properly connected to a load. Make sure your wiring scheme is correct for this application. Just plugging in an open ended RJ-45 to the switch will most likely not result in any power being delivered to that circuit.\