I know barney107 makes and sells them. I would pm her. Here is a link to her store..
Dog Pajamas and Nighties for Yorkie Talkers by T. Bumpkins & Co.
The Sassy Pup has alot of cute ones too!
Pajamas For Dogs, Fleece, Flannel, Thermal Pajama, Pjs for Puppy, Pet, Small, Cat, Sleepwear, Dog PJ
I know I have seen some through Amazon...
Go Fetch and Co. had some but I just checked their website and they don't seem to have very much anymore...
Good luck! I am sure many will chime in..
There is also a thread in the Fashion Forum on how to make your own using a baby sleeper... They end up looking like the 4 leg pj kind..I just made 6 pairs.. I need to post pics she looks adorable in them! And they turned out perfectly!