Slip help

I am having a hard time finding a slip to go with my dress. I got a Private Label dress and it finally came in and I can't wait to go try it on. I now want to buy a slip (petticoat, whatever they are called) and I don't want to get it at the shop (they want $100). I think that I found one on RSVP.com, but the thing is that I don't know what I'm suppose to look for and is it just about fullness or quality?

Anyone else buy their slip online? Can anyone help?

Thanks