Finding a VAT number is a sequence rather than a search, and running it in the right order saves the call. The number is published where a business is obliged to publish it, and it is absent everywhere else. What follows is the order that actually finds it, ending with the step most people skip, which is confirming that the number you found belongs to the business you are dealing with.
Step one: the paperwork you already hold
Every invoice a VAT-registered business raises must carry its registration number, so start with the most recent invoice, credit note or statement. If the supply was quoted, the quotation often carries the full statutory footer as well. On a till receipt from a shop the number is usually at the bottom in small type, and it is what makes the receipt a simplified VAT invoice rather than a proof of payment. Anything you find here is the best evidence available, because it is the number the supplier themselves put against the transaction you are asking about.
Step two: what the business publishes about itself
A UK business trading online is required to publish its identity details, and most put the VAT number in the website footer next to the registered office and the company number. Terms and conditions, the contact page and the privacy notice are the next most likely places. If the business trades through a marketplace, the seller information panel on the listing usually carries it, because the marketplace is obliged to collect it. None of this is HMRC's record, so treat what you find as a candidate number.
Step three: ask, then verify
If the paperwork and the website do not carry it, ask. A supplier charging VAT has an obligation to provide a proper VAT invoice on request and no reason to refuse the number. Once you have it, put it through HMRC's check service and compare the name it returns with the name on your invoice. That last comparison is the point of the whole exercise: a valid number registered to somebody else is exactly what an invoice-redirection attempt looks like, and the number alone will not tell you.
Questions people ask about how to find vat number
How do I find a VAT number when the invoice does not show one?
Check the website footer and terms of business, then ask the supplier directly. If they are charging VAT they must give you a proper VAT invoice on request, and that invoice must carry the number.
Can I find my own company's VAT number?
It is on the VAT registration certificate HMRC sent when you registered, on any invoice you have raised since, and in your VAT online account. There is no way to look it up from the company name alone.
How do I know the VAT number I found is genuine?
Put it through GOV.UK's check a UK VAT number service and compare the registered name it returns with the name on your paperwork. A number that verifies to a different business is the case worth catching.