I am sorry, but no reputable breeder breeds mutts/mixes. Yorkiepoo IS mix. Have you tried local rescues or petfinder.com?
Please keep looking for mixed breeds at shelters/rescues, but not breeders. Those, who breed and sell mutts, are so-called backyard breeders and it is not good idea to support them. They never do genetic testing, so if you buy from these breeders, you might spend lots of money on vet bills in the future
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If you adopt from shelter, at least, you save a life
, not support bad business.