In the Dutch version of this site, I've collected some of the reputable hosta sellers in Belgium and the Netherlands, all but one nurseries or individuals I've bought plants from myself. I can testify that they sell good, healthy plants with a correct price tag.
I'd been thinking of ordering some of those must have rarities from the States. Luckily, in a rare moment of sanity, I decided to get some information about the price tag. So I mailed Gary from Naylor Creek nurseries, and asked him about importing hostas legally from the US to Belgium myself. One hour later I got an answer (great customer service), that shattered all of my hosta importing dreams.
Even if I were a spider, it would cost me an arm and most of my legs to do so. First there is the price for the plants themselves. No problem so far, but than there is a hefty charge for the necessary US phytosanitary certificate to clear the plants for export, the shipping costs, another phytosanitary certificate to import the plants into Belgium. This is only for the major league players, and I'm hardly junior league.
So, I can't give you any first hand advice on where to buy your hostas if you live across the Atlantic.
But you can find a whole bunch of links to specialized hosta sellers on Hostas Galore
How to decide what and where to buy
A great help when you want to buy hostas in the USA seems to be the Hosta Finder by Steve Greene. It's book with where you can buy which hosta with the price.
You can find the order form here.
It can help you:
make up your "wish list", as the Hosta Finder gives information on size and colors (this is useful for the host folk outside the USA as well).
decide where to buy: for every plant you'd like, you get the average price and the price per nursery;
help you place your order: you get the contact information for the specialized hosta nurseries.