Summer cottons or winter flannels, full length or shorties, PJ pants are a year-round necessity!
Make one pair and you will want to make more. If you have a pair that fit nicely, you can use them as a pattern and know they fit. Who Gives a Scrap? has plenty of knits and woven fabrics at good prices.
Just add thread and drawstring or elastic to your shopping list and you are set.