See how to make some cool gift / party favor bags for any occasion. They are made with envelopes. You can make them from nearly any size and type. The ones in the video are made with some surplus greeting card envelopes. Sometimes you can purchase "leftovers" from an office supply or drug store (any store that sells greeting cards). "Left-overs" mean that the cards get damaged or lost but the envelope is left. Just ask the manager if you can purchase them.
You can find bundles of them at thrift stores and garage sales, too. Or look in the back of a drawer or file cabinet. There always seems to be some strays around everyone's house. Watch this video tutorial to see how to transform an envelope into a gift bag.