The Chrom amiibo is now up for preorder on Best Buy
Tiki also has a page, but it doesn’t allow the preorder, only saying “coming soon” so just keep your eyes on the page here. It was available earlier today, so it may be temporarily unavailabe with a “coming soon” placeholder.
We still aren’t entirely sure what they do yet in FE Warriors, however if it’s like Hyrule Warriors we may get bonus weapons and (in Link’s case) a bonus weapon type, although that would go against “classes use one weapon type” which Koei-Tecmo recently confirmed in an interview.
Additionally, the second place winner of the Bond Support snippet show off was also given to us, between Chrom & Ryoma.