Available from any LR parts supplier, prices start at £21 depending where you buy from. These always split after a few years and it is worth buying a new one before having a new windscreen fitted.
Although I have done windscreens in the past I have had mine done by professionals in recent years as I know how easy it is to crack a 90/110 screen.
The first time was when I bought my new heated screen and I just paid the local windscreen firm to do it for me, cost about £10 if I recall correctly. After that it was done when the windscreen was being replaced due to a large stone-chip/crack, I just paid the £40 windscreen excess and let them get on with it. I supplied a new rubber as it wasn't worth reusing the old one as it was over couple of years old by then and as I said, they split over time.