they are printed at the back of the Manx Telephone directory - if you are in Douglas you can get these free from Telecom Office in Victoria St
However your letter will almost certainly arrive with or without postcode

BallaX
Xparish/town
Isle of Man, British Isles

however if you want a reply it is good manners to enclose the postage costs (eg 2 IRCs (International Reply Coupons), UK stamps are not valid on the Island - postage of a letter to the USA/Australia is about 45p/50p (80c), phone calls from the Island, unless you have signed up with a low cost service, are not that cheap but would probably cost less than a stamp offpeak and you could offer to ring them back if they make contact.