I've found the Norton R822 zirconia ones from Pronet Abrasives (via ebay) are worth the extra - they're ~£1 each at 20 off of the 13x451 belts I use on by B&D sander.
They've got a taped joint and seem to keep their grit much better than their own-brand ones.
Ebay link (may well be cheaper direct from Pronet if you're buying lots):
