Hydroponics places list them in their online catalogs.
I don't know whether they are actually in stock at
any given vendor.
Examples:
http://www.discount-hydro.com/produc...d=386&navid=48
http://www.hydroponicequipment.com/c...y2=Accessories
http://www.htgsupply.com/products.as...bcategoryID=24
(This is also where you find net pots for the bottom wick of
DIY earthboxes.)