Listed as "borderline unbelievably rare" by this handy Intellivision game rarity guide.. erm Here
Fathom by Imagic was published 1983, originally designed to be a children's game with (mostly) friendly characters and an easily understood goal to rescue the mermaid trapped in the mystical cage beneath the sea. Seller in Us, ships only to US.
Fathom for Intellivision on ebay.com
Course you may need, well alright then, will need an intellivision to play it on so....
Intellivision Bargains on ebay.com
What's an Intellivision?