Last week I purchased some 4x5 negative holders from my local camera store. He had a box of them sitting under his counter. He gave me a quantity discount when I said that I wanted a dozen. They were $2.00 US each. (Yes, that was Two Dollars each.) The total bill with tax was $26.16.
Sorry about the possible effect on the used holder market. Perhaps there is still merit in supporting your local camera store.