Depends what you want it for really.
You can't go far wrong with Colotech from Xerox (though they have had some very dodgy batches over the years), either direct from Xerox or through a paper merchant. The other "usual suspect" is Color Copy, from memory the manufacturer is Mondi.
Those are reasonable quality, bright white, smooth papers, suitable for general jobbing, i.e. reports.
If you need anything else it gets a bit more complicated.