If your budget is tight and you have a costco nearby, you can opt for their Kirkland brand. The lamb one isn't so bad. And being the costco brand, it's about the same price as Iams (but better than Iams).
When you switched, did you do it gradually? You have to switch gradually over the course of a week or more. If you switch too fast, they can end up with diarrhea. Some dogs need more time than others.