Training Treats
Treats are a cornerstone of any good positive training program. I suggest keeping several varieties on hand: dogs have different preferences, and it may take a few tries to figure out which are your foster mutt's favorites. Rotating treats keeps them from getting boring, and while treats aren't supposed to make up a huge percentage of your dog's diet, you will probably be feeding a fair number of them, so it helps to have a few different kinds of healthy treats to minimize the risk of your foster pup's diet getting too far skewed in any one direction.
Finding healthy treats is much the same as finding healthy food: look for simple, clearly identified meat sources (or cheese, peanut butter, etc.); avoid corn-based treats and anything with a lot of byproducts or digests; avoid anything with artificial colors, sweeteners, or palatants. Treats that are brightly colored and pressed into gimmicky shapes tend to be of poor quality. There are a couple of exceptions, but most of the good treats are just plain-looking pellets or squares, and they're all relatively boring shades of yellow or brown.