You can buy one new, too. At least, a modernized version. They're called Caterhams. When the Seven was discontinued Caterham bought the tooling and continued to produce them, and have further "refined" them over the years. They're bonkers to drive. I took one around the Scottish Highlands over a few days and it's easily one of the best experiences I've ever had behind the wheel.
As others have said, Lotus 7, very nice. Spawned many copy-cats including Westfield and Tiger.
I bought a Tiger Super-Six for my 40th birthday (too many years ago!), great fun although build quality marginal. It had a 2.1 Burton Power build Pinto - that was a gutsy motor in not a lot of car.
No roof, ran it with just side screens. TBH, usability proved too limited by the Scottish weather. Any bikers among us are quite welcome to call me a lightweight for that - but I'm not a biker - I did my share of being freezing and soaking on a rugby field....