I got a pattern at Hancock Fabrics, I love it and its super easy to modify your patterns. However I can't find the Pattern maker or number, I'd check your local fabric store, Joann or Hancocks.
In the meantime I was looking to find the pattern number and found this website:
Dog Clothing Pattern Books.
I know it says for hairless, but it seemed to have a nice variety to choose from. Hope this helps!