I've searched for 3" MDF bases for the Grand Armee troops for ages. Then Russ informs me he's got his from East Riding Minatures (http://shop.eastridingminiatures.co.uk/) and for the princely sum of £1.25 for 6 (or 8 for the artillery bases).
So I've got 60 bases on order - more than enough for the Prussians and British.
I need these as even the thick card I've used to base the troops has warped slightly - so MDF is the only option left.