I was also looking for it last year, and I came to this: there is a list of a couple sources of B1 in this page: http://culturesheet.org/mycorrhiza:terrestrial_orchid_mycorrhizae
Scroll down to "Specificity" section. I think Hardy Orchid Society has them (and a couple other strains). I haven't contacted any of them to see if they can ship it to the US, though.