I want to modify my DIY dolly so that the wheels are on a swivel for use on curved track. But I can't find anything other than lazy susans which won't work with my dolly.

I have this dolly: http://cavalcadeofdeals.com/p/158/Ra...0LO862R99.html

So I need a swivel with a single 1/4" bolt in the middle. I've been to every hardware shop in town and can't find anything.