Android: The Google Play Store does an OK job of highlighting good tablet apps. If you're looking for a better method though, Tablified is a market just for slates.
Since you can't actually distribute alternative app stores through Google Play, Tablified is really more of a go-between. You browse apps in Tablified, and when you find one you like, it links directly to the Play Store. This way, all your downloads are still tied to your Google account, but you don't have to rely on the discoverability options from Google.
Tablified Market - Tablet Apps (free) [Google Play]