Let me take a look around, those're a little tricky to find nowadays. That one is a PMI, stole the picture from PBReview. They came on Rodeo's, Xtra's... a whole bunch of models.
Edit:
http://www.pbnation.com/showthread.p...rails+$8.00+ea
Silver one in there. Going to search some more last night, but that's all I found in 15min. He has it listed as a "chrome Spyder sight rail" for $8.