Probiotics can be found in your normal grocery store. I think they're over near the vitamins.
I use Melaleuca Fiberwise (that links to the product label). If I go daily then the volume is less. Also, drink plenty of water to help move things along. Exercise, even walking, helps move things along.
Personally, we don't use Miralax in our house. We did for one dd and it was a horrible experience for her. It's not a natural product. You can do a google search for some of the issues it causes.