Hi and welcome to the forum.
Depends on what you're looking for, and how much you want to pay, and what distance you'll travel to look/buy. Autotrader
https://www.autotrader.co.uk/cars/bm...ible/petrol/m6 has several at the moment, although the official BMW AUC website only shows one convertible right now. Whilst you may pay more for a BMW AUC, it gives a better degree of protection in the event of something going wrong after you've coughed up!
Good luck.