Toastmasters may help out for that! There is a session at toastmasters that is called Table Topic. During table topic we get asked a question we have no knowledge of before hand, and you have to speak about it for a minute to two minutes. It's a great way to practice it. My interview skills improved a lot after spending a good bit of time in toastmasters. Whenever I interview, I feel like I'm just doing another table topic question
If you join toastmasters for that purpose, I'd suggest joining a smaller club because in small clubs (5-7 people attending) you're more likely to be called to do a table topic each week whereas in bigger clubs you won't be called upon that frequently due to the limit of table topics per meeting.