I planted my peonies a week ago, although I realized I had run out of plant markers for them.  For the last few years I have been using Kincaid Gardens 100% Stainless Steel Plant Markers, and they have been awesome.  They don't rust, are very sturdy, and definitely last longer than any other plant markers I've tried.  So last week I put temporary labels on the new peony plantings, and today I'm replacing them with the Kincaid Gardens plant markers that I just received in the mail this week.  They are definitely more expensive than other plant markers, but they also last!  Here's a link to their site if you're interested...  
Kincaid Gardens  I also use Brother P-Touch Extra Strength Adhesive Tape for printing the peony labels, which also last for years.  I haven't had any of those fade or peel yet, and I've been using those for a few years now as well.  Happy planting!