The 15400 and 15300 use the calibre 3120, which is an in-house AP movement introduced 10ish years ago, not a JLC ebauche like the 15202. Most of the watches that use a 3120 probably haven't even seen their first service, so it's unlikely any of the movements are available anywhere but in complete watches, and the Audemars factory itself.
The dials and hands exist from 2012 on, and again are almost certainly only in watches that are still in their initial warranty, let alone have been serviced or replaced with furniture dials.
Your best bet for frankens are ROOs (which often spawn OEM parts when 'blinged' out) or other older models, which have had parts replaced by AP and the original parts become available for reasonable prices in the grey market.
2121 movements are relatively common, as AP used them for a LOT of watches in the 60s and 70s, and the same exact movement was used by Patek and VC for many thin watches. A reasonable price for one is around 2500usd, or if you can get a vintage watch for a good price, you can take out the movement and sell the gold case for a discount.