We sent out our save the dates a month ago and we were originally going to get magnets made, then decided we could make our own paper ones. But then we went into our local craft store and found Magentic Photo Paper. It was $12.95 and had 5 sheets and we fit 8 STD's to a page. It is super thin (at first I didn't believe it would work but I tested it out in the shop) and it goes really well through the printer. I wrote magnet on the envelope so people didn't put it on any computers and got comments back that it took them ages to find the magnet lol.
It was fun to make them ourselves and they looked really good. Just thought I'd share if anyone was looking for a cheaper (and more fun) option for their STD's.
PS. The store is called Spotlight in OZ, I don't know what the equivalent craft store in the US would be.