Iasked this question earlier in the week and was told the post office and www. able2travel.co.uk. The post office looked quite good. You have to ring the other one if you have a medical condition.
We got a good quote for my 84 year old Mum who has had a couple of TIA's from www.staysure.co.uk . You can't take it out until 90 days before travelling, so will use them later this year.