Contact your local rag merchants in yellow pages.
Or find out who your local hotel laundery services are and contact them, ours is Sunlight, every so often they replace all their hand and bath towels and the old ones generally go on contract to the rag merchants, but, if you can find yourself a contact at the laundry you can get them before they cut them all up, we just got a pile of 200 + handtowels for £20. use them mainly for window cleaning as sill cloths.
I did buy some of the ones from Costco a while ago but found them to be a bit on the small side