You can make your own. Simple starter pattern here. https://www.backyardchickens.com/articles/how-to-treat-sour-crop-and-impacted-crop-and-how-to-know-which-one-youre-dealing-with.73607/
If a crop isn't flat and empty come morning, it's safe to assume a crop disorder of some sort. The next step is to determine which so you then know how to go about treating it to get it back to normal.
A squishy, liquid-feeling crop in the morning would indicate a yeast infection, especially if...
Unless you check the crop last thing before roosting and first thing in the morning before permitting access to food and water, you really can't know for sure what condition the crop is in. Here's an article I wrote to help you figure out what's going on...