As the subject says, I am unable to find a link. Retail version.

Twin Islands (and Crateus, for that matter) is MAN-only.

If you're referring to d/ling it for MAN, it's here: http://www.massiveassaultnetwork.com/download.php

I just realized I posted this in the wrong section. Sorry about that.

I am referring to the boxed version as I just received a challenge on this map. If I received a challenge from another retail owner then I should be able to play this map?

well..maps like Crateus and Twin Islands are for MAN subscribers only so owners of old MA version can not play on them and can not send such challenges,
If some subscriber challenged you on such map you not own - he made mistake cuz you wont be able to play - so you should just reject such challenge.
The possibility to send illegal challenges is a bug which will be fixed ...i hope soon...