If yours is anything like mine, this piece of rubber is usually well past its sell by date but does not appear to be readily available from the usual suppliers. However, you will probably find that the rubber strip, which goes in the channel at the bottom edge of the door itself, and which seals against the kick plate, will do the job just as well. It has the same T shape profile and, once the old rubber has been removed and the channel in which it sat has been 'de-cripmed' , cleaned and lubricated with a small amout of washing-up liquiid, the new channel fits in perfectly and just needs trimming for length.
If it helps I'll post a few photo's if asked but I've found it works very well indeed

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