If you need a copy of a document certified as a true copy of an original ('a certified copy') then you may need help from a practising Solicitor or a Notary Public.
Certified copies may be required for example by banks in connection with opening a bank account, by prospective employers as part of the recruitment process or by immigration authorities in connection with travel visas, work permits or immigration. Often certified copies are needed if you are conducting legal business outside the UK.
For a certified copy for use within the UK you usually need to see a Solicitor. Information about finding a solicitor is available from the Law Society.
For a certified copy for use outside the UK you usually need to see a Notary Public. More information about finding a Notary Public is available from the Notaries Society.
Fees vary and you should agree them with the Solicitor or Notary in advance.
Affidavits and statutory declarations are formal statements which the maker is swearing on oath to be true. They are usually made in front of a Solicitor. To find one please refer to the link above. Fees are fixed. At the time of writing they are £5 for the statement itself and £2 for every accompanying document.
You will NOT be given any advice on the legality or wisdom of what you are doing by the Solicitor or Notary. Their role is simply to ensure compliance with the formalities. You will need to employ your own lawyer to advise you.