What expansion tanks do people use on their cars? Aftermarket or OEM from a specific manufacturer?

Mine has one from an MG ZR but the seal in the cap has gone. I know I can get a new cap but I could also replace it with something shiny

There's tonnes of different alloy shiny ones. Just pick one you like with enough inlets/outlets.

http://www.alloyracingfabrications.com make some nice ones. I have a fuel tank and oil catch tank from them, but I'm not using an expansion tank this time.

Renault Clio mk2 looks neat cheap as chips. easy to mount loads of new ones on eBay, for £12 or so.