Here is a pattern I used to start
http://cgi.ebay.com/S3925-Pattern-Pe...QQcmdZViewItem
But I ended up improvising and creating my own. It's a good starting place though! They look a bit short, but length is easy to adjust.
Pj's are always hard for us too, Mckenzie has about 6 pairs and so far none still fit her perfectly, she can always squirm out. I've started making her fleece nightshirts that seem to keep her really warm because the 4 legged ones just don't do it for her.