If you want a really inexpensive one I bought one at the Motherhood store. Yes, the maternity store. (they fit people who are not pregnant as well as they STRETCH ) I bought the smallest they had and what was great about it was it had a wide strip of lace at the bottom that you could cut tiers off to make it the length you needed it to be. It was so reasonable priced and really fit me perfect.
If not, this is a great online store:
http://www.herroom.com/slips.aspx
They have all levels of quality and such a selection.
IMO, a great slip comes in handy!