Pond liner

Hello I’ve had a pond for over 20 years, and I’m thinking of changing the liner early next year and making it bigger Now what liner should I buy, the one that’s in it now, it’s not rubber, yet it’s lasted well over 20 years So I’ve been looking at rubber, but the costs are a lot Does anyone have recommendations, and also where is the best place to get it from I’ve attached some photos of the new bigger pond idea and my old pond I’m going to build up so there’s any extra 2ft of water at the front It’s about 3600 litres now See less