Where can I get more Manakete materials?

I'm about 50 hours in to Fire Emblem Warriors with most of the history mode maps complete (not counting the extra purple portal levels). I have slowly been able to upgrade every character through the Crest Market, though I have a notable issue upgrading Tiki.

The main issue is she requires Manakete scales and tails. Not to mention that lots of those are required if I want to upgrade everyone's defense against dragonstone attacks. Is there a map I can grind to get a lot of these types of material?

The Chapter you unlock Tiki in and a few after that contain many beasts. These drop the Manakete materials. You can repeat any story chapter you've completed in free mode to grind away.