Welcome Martha! Try this link for Collectors Nursery. I ordered from them last year and was very happy with their customer service. I got a couple of unusual plants and have more ordered for this year. They have quite a selection of epimediums. Good luck!

Here is a link that might be useful: Collectors Nursery

I always get a bit uncomfortble with questions about recommending businesses. Mostly because I am in the ornamental plant business (over 27 years) and want to say come to my place and I'll sell it to you. However, due to reugulations on the forums, I cant do that.

Anyways good luck finding your plants, I adore the bishops weed which you are referencing to. Georgousfloweers that resemble the crown of a bishop and the tipped red edge of the large leaves make for interest after the flowers have past. Be sure and plant this in shade to part shade, I am not sure how the Boston Flower Show showed it.