Because of the work needed and the cost of a phytosanitary certificate few European nurseries are interested in shipping to the US, so it would probably be both easier and cheaper to buy ferns from one of the Chinese nurseries. You will also need to apply for an Import Permit, but it's free and just a formality.

Here are some links:

http://adhocplants.com/
http://www.pottertons.co.uk/ - Both are willing to export and have great plants, but I don't know if they carry any ferns.

http://home.no.net/chenyi/ - Several rare Chinese ferns, one of Plant Delights' suppliers.

What the previous post says is true, but there are some UK nurseries that will ship here if you're willing to bear the trouble and expense.

http://www.burncoose.co.uk/catlg.cfm?plant=HARDY%20FERNS

You can check for other sources by using the free RHS PlantFinder to locate the plants you want, then check their detail listings or their websites to see if they will ship here. Most won't, so this method is inefficient, but sometimes works.

Here is a link that might be useful: RHS PlantFinder

Do you know Surry Gardens in Surry, Maine? They have quite a few unusual ferns. Also the Garden Institute at Merry Gardens (formerly the famous Merry Gardens, Camden, Maine) may well have some of Mrs. Ross's unusual ferns.