No there are very little standards for newsletters.
If you want a banner looking ad on your newsletter (or just a banner) simply resize your image to the proper width -> 600px for example. And keep your ratio.
Newsletter are usually 600px wide (in order to appear correctly under most circumstances). If you can't put a standard width advert banner in your newsletter, it's simply because that standard wasn't decided for this context. But a resized banner will look great in your newsletter, as long as you keep the image ratio you'll be fine. (you can use any tool, from paint to photoshop to resize an image)
Hope this helps