For people still looking for this deal, search ebay for "640sfp28" cards. These are HP branded Connectx 4 cards. They're very easy to cross flash to the standard Mellanox firmware.
Standard profile
www.ebay.com
Low profile
Standard profile

HPE 817751-001 SFP 10 Gbp/s Network Card for sale online | eBay
Find many great new & used options and get the best deals for HPE 817751-001 SFP 10 Gbp/s Network Card at the best online prices at eBay! Free shipping for many products!
Low profile

HPE 840140-001 Ethernet 10/25Gb 2-port 640SFP28 NIC Adapter Card Low Profile | eBay
30 day warranty
www.ebay.com