An ebay seller has posted some 3x rear PCIe cages for DL180 G6 HPs, looks like it comes with a 1x x16 riser. $89 shipped with best offer available:
HP DL180 G6 PCIe Riser Cage 502428 001 507258 001 490450 001 | eBay
HP DL180 G6 PCIe Riser Cage 502428 001 507258 001 490450 001 | eBay