how do you get creatu from Uldavi

You get them through doing Kir's Quest (under games > quest). The quest involves handing in specific creatu that kir asks for, and you can use the points to buy things from the prize shop, which includes uldavian eggs. It's quite a difficult quest though, as he often asks for high colours like albino. It's probably easier to start saving up tu to buy the creatu from another user :)

Do you know who sells this kind specifically?

Hey pollen, you can use the creatu search under the Information tab to search for specific species in rancher shops. Just choose the desired species, set location to Rancher Shops, set minimum price to 1 and order by price ascending to view the cheapest ones first.