The Isle of Man Government Registries site has all the information you need about the legal side of getting married here in the Isle of Man.
It includes a list of the places in the Isle of Man which have been granted Approved Place status to hold marriage and civil partnership ceremonies. Note that most places will expect you to pay a fee for getting married at their establishment on top of the fee you pay to the registrar to legalise your marriage.
The Isle of Man Government website also details the practicalities of arranging your legal marriage in the Isle of Man. Any questions, e-mail the registrar – they are very helpful!