I would recommend looking at the types of dogs she does get along with in parks. For instance, Thor likes other yorkies, and he likes daschunds, I think because their noses are about the same distance of the ground as his is.
It also might just be temperment - she just needs a nice, calm buddy. You could go to a shelter with your current yorkie and try her around a few other dogs.
I wouldn't get a dog that is too much bigger than her, since he might accidentally hurt her while they are playing.