Natalie j, honestly I don't know if H sells the knotting cards bc I've always gotten them for free. They usually throw in a pack when I buy a scarf.
IMHO, I don't find them particularly helpful myself. There is an app you can download with videos of a few knots. They update it with new knots occasionally.
I personally find MaiTai's website a fantastic resource for a large catalogue of scarf tying techniques!
Here is her website (look along the right column for knots sorted by different categories, including knots by scarf format!)
http://www.maitaispicturebook.com/?m=0
Enjoy!!