Your niece is asking for the lil kinz pig. It retails for $9.99 at Hallmark stores. (The original Webkinz pig is the one that is retired.) The pig is pink and looks like a little piglet.
The reason you are having trouble locating it is because it is the official "Pet of the Month" on the website. Once they register the pig, they will receive bonus items online for having the special pet of the month.
I would advice you to look on ebay to get a feel for what you are actually searching for at the store. Look up Lil Kinz pig.
Seeing as these are hard to find, you will probably have to go to a lesser known place that sells them to find it. These stores mark up the price a few dollars, but have less Webkinz traffic. (Limited Too, Cracker Barrel, Libby Lu, etc.) Even our local JC Penney's sells them!
Good luck finding this. By the way, the reason I know so much about this item is because I am searching for one too!