We used just Emma's kibble as treats when she was young. She didn't know the difference
I also used cheerios-if she doesn't like the plain ones, I think Honey Nut would be ok-I've heard they like the sweetness. My two also like frozen green peas, small slices of baby carrots and bananas, and bits of cheddar cheese.
If you want a commercial treat, try the Itty Bitty Buddy Biscuits by Cloud Star-they are just the right yorkie size, and you can even break them in half so they last longer. They come in yummy flavors like peanut butter, molasses, & sweet potato.