I'm so sorry to hear of your little Tansy -- bless her heart.
Instead of buying a 12 week old puppy, I would recommend you buy a 5 or 6 month old pup. Or better yet, a lot of breeders sell pups around 8 or 9 months old (maybe even a tad older) because they turned out to be too small for breeding. That may be your best bet because it's just too hard to guess what weight a puppy will be when full grown. The breeder can only estimate the weight ... not guarantee it. You're safer getting a slightly older puppy. And really, 6 or even 8 months old it still YOUNG when compared to how long these little ones can live.
I wish you all the best of luck in your search for another Yorkie. Please give Tansy kisses from me and my crew. What a little angel.