Hammocks are available from Dreamweezle Hammocks & Beds. They are excellent quality and are made by Jo Jeynes to raise funds for the rescue. My ferrets have been testing them out and are well satisfied.
They come in all sorts of shapes and sizes and can be fleece or cotton whichever you would like.

Ferrets About, Ruth and Eric (husband and wife) run it. Ruth makes the bedding ie sleep-and-peek hammocks, multifunction bags, carrier hammocks etc.
Eric deals with most of the orders, but Ruth takes phone calls and deals with packing orders etc, when Eric is away, so it is very much a joint thing. You have to check out their website to see the lovely selection of ferrety things that they have for sale.