Holy crap...going by this thread, I got super lucky. My amiibo adventure:
I started collecting amiibo cards back when HHD was released, and a lot of the series 1-3 cards were very cheap. This was before anyone knew there would eventually be an update for New Leaf. A friend of mine sent half of series 1 to me for free. During that time, I ended up ordering the complete Japanese series 2 and 3 sets for $55 and $80 dollars each from Amazon. After the update was released, I got a whole bunch of series 4 cards from ToysRUs during their 'buy 1 amiibo get two free' sale. I got some of the rare series 4 cards (Lolly, Stitches, Pietro) from there. I ordered a complete Japanese series 5 from Ebay for $80. Thanks to an unfortunate mishap with my local post office distribution center, I actually ended up ordering two series 5 Japanese sets, and sold one of them off, making my money back.
The Sanrio set was a complete surprise: when the the prices immediately skyrocketed on ebay after their release, I started searching around the net. I tried to get them from the game.uk site, but ended up having to cancel my order and get a refund. After searching around some more, I ended up bumping into a site called storenvy. To my surprise, someone on this site had a storefront selling all 6 cards for $12 total, included shipping. I ended up ordering 2 sets of six for both myself and a friend.
My sister had some points saved from a promotion on etsy, so she got me got nine custom cards for basically free using her points. I wanted them because its just easier for me to store cards rather than full amiibo figurines.
For the amiibo festival cards, I used TBT's amiibo card post office, and had wonderful experiences trading with other members here, LuciaMew and samyfav.
From there, finishing the collection was a breeze. Uusing both ebay/reddit, I bought or traded the few singles from series 1 and 4 that I still hadn't collected. For the 'rare' tier 1 cards, I used the 'rare' tier 1 dupes that I got from the ToysRUs sale for trading on reddit. For the tier 2-5 cards, I bought them as singles on ebay since they were less that $2 each.
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To OP and everyone else in this thread: If you can, trade your way to the cards you're looking for. It's better to trade than to buy at an outrageous price. In my experience, I can almost guarantee that someone out there has a card you're looking for if you have a card they want, and outside of standard postage, you won't have to pay a penny extra.
ToysRUs is a great source for the time being. Even if you don't find the cards you're looking for, you can trade the ones you get and don't want. I remember trading for Lucky, Rosie and Erik with dupes of tier 1 cards I got from TRU. Aside from standard postage, I didn't have to pay any more for popular cards than what I already did, because I had cards someone else wanted. TRU right now seems to be a reliable source for cards.
I didn't let language barriers stop me. My series 2, 3, and WA are all Japanese because I know all cards have the same info and work the same way, so language didn't bother me. That may be an issue for some people, but if you're willing to step outside of the language/region comfort zone, you can pretty much get whoever you're looking for pretty cheap. Besides, the way I look at it, the Sanrio cards don't look to be coming to America anytime soon, and they're not NA region. Now I will admit that for the sake of uniformity, I do have all cards in a single set be from the same region, but overall, my sets are from every region: Series 1, 4 & AF are NA, series 2, 3 & WA are JP, and Sanrio is EU.
TBT's amiibo Card Post Office was extremely helpful. Other places like moderated facebook groups, and reddit's amiiboswap sub are valuable and safe places to trade. From my experience, both the amiibo card post office and reddit's amiiboswap sub have a ratings system and active, helpful moderation. With those two places, I never had a trade go bad or wrong, and never had to deal with a scammer. With facebook, it depends on the group that you join. The group I traded with is now defunct, but there are plenty of others. Just make sure that whatever group you join is active, and most importantly moderated, with ratings for members. The only place I do NOT recommend you trade is tumblr. Tumblr was the only platform that I used where I ended up being scammed. I didn't really care since it was a like a tier 4 dupe, but if it hadn't been, I would have cared a lot. Stay away from tumblr trading or use at your own risk.
As for Sanrio, it's gonna be super hard, since they're like the most popular ones, but that's what I thought when I started looking for them. If anyone is interested you can try that storenvy shop I mentioned earlier. I don't wanna link it since I'm not sure whether or not it's against the rules, but just google the term 'storenvy', and search for peachybearsbabyshop. She's the one who sold them to me. Currently it looks like she's out of stock, but you can message her to see if/when she'll have more.
It took well over a year and a half, but I ended up completing my amiibo collection a little while ago in summer of 2017. I don't have the CP cards, but to be honest, I'm not really chasing after those. I hope this post doesn't come off as grandstanding. It's just that I had a ton of fun trading cards, and I want the experience to be as interesting and fun for other people as it was for me.
Happy card hunting.