Check the manufacturers website. The often sell spare parts.
I'm assuming in your case its a generic enclosure, so -
Look online for brand that also sue the 4-pin power connector eg Thermaltake, Antec, Coolermaster, Vantec etc
The 4 Pin connectors are universal. If the pin-outs (+5 GND GND +12) are the same, then you could use one of these to replace your missing one.