As the payments are collected directly by the accommodations, they are the ones issuing the invoices.
To request an invoice, you can contact the accommodation administration directly - their contact details will be provided in the reservation confirmation.
Alternatively, please reach out to the Customer Service of booking.com and they will request the invoice on your behalf. In this case, you would save your time if invoices are needed from multiple properties.
You can find a local number for the Customer Service here: https://www.booking.com/content/offices.en-gb.html
Please note, that not all of the properties are VAT payers.