I am in the saaaaaaaaame boat as you lol. but 17 packs?! dang i admire your tenacity. I was the same way and was so bum to not get any rares lol. i was salty. but thats why now i search online. I stalk amazon and stuff until those welcome amiibo packs go on sale or auction on ebay...individually.
GameStop only sold Series 1-3, and if your local store has any in stock, they'll be discounted. Also check Best Buy, Target, and Walmart. I bought packs from all of those stores, but I got them when they were released. I never looked for Welcome Amiibo (which I heard has been discontinued) because I bought my set on eBay. I also bought the set of six Sanrio cards on eBay. You can easily find singles on eBay and Amazon.