I haven't ordered any certs recently so I don't know the current cost, but I used to write to The Chief Registrar, General Registry, Finch Road, Douglas, IoM. The cost used to be around five pounds sterling for a full cert plus search fees of #1.50 for each year. They sent the certs and the bill, asking for payment in sterling, rather than requiring payment in advance like most GROs. Perhaps try posting your request to the above address, including exact names, places, and dates if you know them.
Civil Regt of births & deaths started in 1878, and marriages in 1884. Earlier than those dates it is best to follow Donna's suggestion.
Sue