Last I looked LEGO Island (and the rest of the classic titles) aren't in print anymore, but if you can I'd definitely suggest getting a physical copy - a quick ebay search brings up (for the UK at least) quite a few copies and nearly all of them are under a tenner, but if you've got a few charity shops nearby it may be worth checking there too on the offchance someone's donated a copy.
Some folks are also properly into just collecting the early 90s big box versions, but if you just wanna play the game then any of the newer releases will do - they all have the same game on the disc after all, it's just the packaging that's different (I still swear by my RE:FLEX copy from my childhood!).
Now if you do decide to go the download route instead, we can't help you there as per the forum rules but I'm sure a bit of browsing through google should help you there.