Yatsuhashi is cinnamon flavored dough with sweet bean paste inside. And mainly they have two different kinds; soft (raw) and hard/crunchy cookie style. Both are called Yatsuhashi, but they are very different. Soft & raw Yatsuhashi (triangle shape) has bean paste inside and when it is in your mouth, the dough melts quickly but flavor of cinnamon remains for a while. On the other hand, a hard/crunchy Yatsuhashi has a rectangle shape and it is harder than normal cookies. It still keeps the cinnamon flavor and it might go well with a cup of coffee.
Most people prefer the soft style, because they tend to have more flavors available, for example one has chocolate paste and another has strawberry jam inside instead of the standard sweet bean paste. However, these go bad more quickly than the hard counterpart (usually about ten days). So it is a good idea to buy them one day before or on the day of departure. As for a place to buy Yatsuhashi, it is almost ubiquitous. Even some convenience stores sell them.
Here’s a picture of the soft Yatsuhashi: