Can't tell from that picture of your rear hub, but Exa-Drive would be my guess (check link below). 8-speed Campagnolo cassettes are getting harder to find, but I have a NOS 13-26 Exadrive 8-spd cassette available FS. My ad has fallen off the Classic and Vintage Sales forum, but it's still available, along with a bunch of loose Campy Exadrive cogs. PM me if you're interested.

Cassettes and Cogs - Branford Bike - Seattle/Bellevue - Campagnolo Pro Shop

You'll have better luck with your questions over in that forum or the C&V forum itself. Lots of very knowledgeable folks over there.

Any good SRAM or KMC 8-spd (or lower) will work fine and should last several thousand miles with decent care. My wife's Campy Chorus bike wasn't as happy with a Wipperman chain and I've been less impressed by their life. And FWIW, I converted her bike to Shimano cassette hub for better cassette availability and used these spacers from Wheels Mfg to make the shifting work properly.

8 Speed Conversion REVIVED LIMITED RELEASE