Try fse.anthro.fr which is a search engine for sofurry. You can use an unlimited amount of tags or words in the search and results are ordered by the amount of words that you searched for that appear in that story.

For example: transformation female (dog or bitch or collar or leash or bark or "good girl")

Will give you female transformation stories ordered by the prevalence of words such as dog, bitch, collar, leash, bark or “good girl”.