I too was looking for a long shallot to grow from seed and came across a variety in the simpsons seeds catalogue. Its called Cuisses de Poulet du Poitou. I will be trying it for the first time in the coming season, so don’t know how it performs. The round variety Matador grew well for me last year and is storing well.
Last year i planted my own bulbs of Longor and they grew well and are keeping well apart from a little mildew. Conservor F1 from seed grew well too and is eating well now.