I use Flecto Varathane... I think they renamed it as Diamond Varathane, LOVE IT!!! You can even Rebake your piece after it has been applied! It is a water based acrylic FINISH (not a varnish, polish or floorwax) for wood, paper, fabric, and other media (like our polymer clay!) that is found in many hardware stores.
There is also a retail source finder section on the official Flecto website
http://www.flecto.com/. If you want to know about availablity in your area, ask them. The effectiveness of this product as a longterm finish for polymer clay has opened the way to exploring making my own stains by adding just a touch of colored acrylic paint.