Does anyone know a place where I can pick up one of those four pin A/C adapters for an external hard drive?  I have a really nice external HD, and I may have misplaced the A/C adapter for it (I haven't really looked around too hard for it yet).  I did a brief search over at new egg, and couldn't find anything.

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.

Try this site and good luck.
 
http://www.mpja.com/

go to 12vAdapters.com. They should be able to help you out.