You could use a local notification that appears when your app is running.
ToastTemplateType toastTemplateXml = ToastTemplateType.ToastImageAndText01;
XmlDocument toastXml = ToastNotificationManager.GetTemplateContent(toastTemplateXml);
You'll then need to populate the XML returned by GetTemplateContent
<toast>
<visual>
<binding template="ToastImageAndText01">
<image id="img" src=""/>
<text id="txt"></text>
</binding>
</visual>
</toast>
Supply the content of your toast in the XML DOM. The image is relevant only for Windows 8.1.
Specify it's launch parameters
((XmlElement)toastNode).SetAttribute("launch", "{\"type\":\"toast\",\"param1\":\"1\",\"param2\":\"2\"}");
Create the toast object:
ToastNotification toast = new ToastNotification(toastXml);
and finally display the toast.
ToastNotificationManager.CreateToastNotifier().Show(toast);
Also, If you want to use a third party control to display the toast then you could consider writing a Windows Phone 8.1 Silverlight app.