I keep tabs on amiibo tracking sites and jump when someone posts that amazon has them for sale. Otherwise someone in my town has been buying up all of the cards from different shops.
Target.com has them in stock! Just ordered 3 packs for $8 and free shipping! I used a red card for free shipping and had a $10 off coupon for any online order.
My sister and I were *extremely* lucky in getting as many packs as we did. One Gamestop we went to only allowed one pack per customer but at another Gamestop (that we are regulars and the employees recognize us) let us buy as many packs as we wanted (we only purchased three each). And we cleared out one Walmart of it's entire stock of cards over the course of three weeks because no one was buying them AT ALL. Yet at other Walmarts, they had zilch.
So in my personal experience, getting these cards has been a crap shoot.