Marth and Rosalina: importing and online scalpers. That's it. Well, you might find Rosalina in a Target store if you're prone to experiencing miracles, but it's extraordinarily unlikely. Ike, you could try waiting for the next batch coming in May or pray he pops up online somewhere, but if you want him right now for sure, it's the same case as Marth and Rosalina - scalpers and imports.
As for Wave 4, it's not available for pre-order here except for at FYE in-store (which may have ended and seems very likely to fall through anyway). Follow this thread and r/amiibo a lot and you should catch when pre-orders go up, but there are also services you can subscribe to such as NowInStock.net and a couple of GAFfer run resources. When they do go up, I'd recommend pre-ordering ASAP, as later orders (particularly with the last wave) have been delayed and/or canceled all too frequently. Avoid pre-ordering from Best Buy and TRU if you can as they've had numerous issues, everyone else is pretty much equally reliable now-a-days.
