As bransoj has said, going direct to Vodafone should be your preferred option for peace of mind. However if this isn't possible, then have a look on ebay as there's plenty of sellers - obviously ensure you're dealing with someone with good feedback. As an example I recently got my iPhone XS (on EE) unlocked through an ebay seller without any issues - phone is less than 6 months old which meant this wouldn't be possible if i went through EE directly.