any good hardware store should have a selection to choose from. usally best to have two fixed and two turning and get at least two of them with parking brakes.
I was thinking like at least 6' of rollers 1" to 2" apart on each side... I was think finding some industrial surplus store that may have something like that?
You can almost make your own. Its just a matter of finding a bearing to fit whatever ID of the tube you want to use. I would go more than 2" apart though.
I've used ones similiar to these. http://www.rockler.com/product.cfm?page=10537 They're inexpensive and pretty handy in the field since you can adjust them up/down and closer/further to match your table and material.