Where can I cancel my newsstand subscriptions?
Go to Settings then choose Store.  There you will find a list of your current subscriptions, which you can turn to "off" if you no longer wish to subscribe to a given product.

Some users have indicated that there are still issues, such as unsubscribing, then going back into settings --> store and finding they are still subscribed, but this seems to be rare, and I expect it will be resolved.

There are also usually contact options to reach the developer of the product in the newstand store, just like contact options in the app store.  You can contact the developer directly for support if their app doesn't seem to be successfully unsubscribing.

Most newstand subscriptions should have an interface to unsubscribe in the app itself, or under the app in Settings.  

Due to the way subscriptions work in the App Store, though, a developer would have to add a non-recurring subscription option to their app.  I imagine most won't - they will only offer their subscriptions under the app store's auto-renewal process.

You should contact the developers directly if you wish them to add a non-recurring purchase option.