I believe you mean the horizontal one, the connector is from SAMTEC P/N MEC1-160-02-S-D-EM2.

I don't think Digi-Key has it in their catalog, but they list the vertical one P/N MEC1-160-02-S-D-A
(Digi-Key P/N SAM8121-ND $11.99/ea) it seems that they are currently out of stock but you can obtain
it directly from Microchip Direct CON0197 for $12.95/ea.

One Forum Member (zardoz1) did something interesting putting together two of the
vertical connectors back to back to extend horizontally the PICtail edge connector
(same as the I/O expansion board) of the Explorer 16 with a prototype PICtail.



My .02