The D5100 manual can use a Nikon MC-DC2 cable, which is specified on D5100 manual page 207.
Says Reference Manual, can be downloaded at
http://support.nikonusa.com/app/answers/detail/a_id/17325
Searching Ebay for Nikon MC-DC2 will find some some very inexpensive import copies - my experience they work just as well.
Also page 207 (under accessories) shows the ML-L3 remote shutter button, which is about $17 at Amazon and B&H, etc. This will do the same job, even from a greater distance, but it has to be more or less in front of camera.
Text search that Reference manual for ML-L3 (a few places) to find usage specifics.