1 option is to simply buy the following silver chloride pellet from WPI: http://www.wpiinc.com/index.php/EP2.html
Or, to make a nice reference electrode for acute recordings, we use:
A silver wire (such as can be obtained from Axon Instruments; a lot of labs have this around)
Belden Test Wire part 8890 010100 (Rubber insulated cable ("slip coat") that is very flexible; a spool can be ordered from Allied electronics part number 214-3500
A gold pin of your choosing
Optional but recommended: heat shrink tubing and a heat source
First, if the silver wire is covered in insulation (very likely; you may not even be able to see the insulation if it is clear but it is probably there), use a razor blade to gently scrap the insulation about 0.5cm from each end.
Next, cut a length of the Belden wire to the appropriate length and strip about 1cm off both ends.
Solder the silver wire onto one end of the wire. Solder the gold pin on the other.
If you are using heat shrink tubing, put a little over both ends and use the heat source to cover the solder joints so they are mechanically more stable.