Hope someone can point me to a supplier.

I believe this is one of the parts that have not been available for some time. Others have suggested using a ball joint rubber boot as an alternative. Another option is to try to restore what you have using the oil of wintergreen method promoted on numerous old vehicle and bike forums and sites.

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ex-NSW Police 1970 MK II Cooper S
VMCI #43

My understanding is the same as "winabbey's" regarding modern sources.
Since the main purpose of the seal is to keep water/dust out of the boot area plus a slight lift of the petrol tank, could you try packing several toilet cistern suction washers stacked together to get height? They tend to be better quality than after-market mini rubber parts!