from the purchased list: And not just iDOS; I was able to download Tris, Star Wars: Battle for Hoth and the non-deluxe version of SimCity (to name a few.) But not everything I bought is available: neither Nescaline nor StoneLoops! of Jurassica are in my purchased list. Oh well, that's why I backup everything locally. Also, these don't show up in a search. The only way to find them is to browse your entire purchase history.
I backed up both my copys of iDos as well as GT racing which Gameloft turned freemium same with a few games that have just vanished from the store for no reason like Flatspace. Suprisingly they all seem to be in the purchased section now as well.
You need to have already bought it. It was only in the App Store briefly before Apple removed it, that's why it is surprising people who bought it can still download it from the purchased section.