Waterproof Thread Sewing Outdoor Chair Cushions?

Sewing outdoor chair cushions? - waterproof thread

I want to make my chair cushions for garden chairs outside. Which material is better (and I can online @ Hancock or Joann or any other Web site to receive)? In addition, a special piece of foam should I use? A special thread? Waterproofing is a problem? Anything I should know? Thank you.