Unfortunately, we don't currently have a way to offer alerts for stock status changes. This is a feature we'd like to add in the future, though!
If you're interested in keeping an eye on an item, we recommend bookmarking that item and checking back frequently. Additionally, you could try using a page update alert browser extension that is compatible with your browser. Page update extensions will alert you when pages you're watching have been changed. However, these extensions are not affiliated with HobbyLink Japan and we cannot guarantee their reliability.