And:
 
http://www.kvraudio.com/forum/viewtopic.php?t=331065
 
The above seem to be samples that you would need to map into an instrument.
 
Bob Bone
 

And the Garriton Pocket Orchestra seems to have a guiro in its libraries, though I am not sure what version of this has it.
 
Bob Bone
 
 

Wow! Thanks so much, Bob.  You are awesome.

No need to thank me - Google did all the work.  :)
 
Bob Bone
 

Here is something that might be quite useful:
 
http://computermusicresource.com/GM.Percussion.KeyMap.html
 
Apparently, the General MIDI spec for percussion includes a short and a long guiro sound.  That will certainly not compare with an entire instrument of guiro sounds and articulations, but it DOES mean that any instrument with a GM drum mapping should have at least some kind of approximation for those 2 sounds.  So, check out TTS-1, and maybe even Session Drummer, etc.
 
Bob Bone
 

Thanks again, Bob.  I thought I had encountered before, but couldn't find it.