I would buy a bench and have it repainted/repadded. People often think white is just white, but there are many shades of white around. I would get somebody to colour match a paint to your piano, and then get it sprayed. Having a slightly different shade of white next to your piano could be very unpleasant to look at - more than you might think. You could take a panel of your piano along with your bench to an auto-body repair shop and I'm sure they could colour-match and spray your bench with a hardy, glossy polyurethane paint. That's what I would do in your situation.