Not difficult at all to get Amiibos if you preorder. All of them were available to preorder for weeks on multiple websites. Same for the retailer exclusive ones (except for Rosalina).
Even now that everyone has Amiibo fever, it took several hours for the latest SMB line to sell out.
Nintendo said before the Amiibos came out that some of them were going to be limited, but even then, there has been plenty of time to pre-order Wii Fit Trainer, Villager, Marth, Little Mac, Pit.
Also, Wal-Mart does not have a retailer-exclusive amiibo.
This is blatantly false. "Hours" is not an acceptable amount of time for something to be available for pre-order. If you are away from a computer or working, there is literally no opportunity for you to buy these amiibo.