With the introduction of the Ryzen 3000 series processors, the older Ryzen 1000 and 2000 processors are no longer in production. Ryzen 1000 and 2000 processors may still be available but once inventories are gone AM4 users will need to update their BIOS and chose one of the Ryzen 3000 series processors.
All motherboards except for the A320 have updated the BIOS to accommodate the Ryzen 3000 series processors. The upcoming B550 is still unclear as to CPU compatibility. Given its PCIe 3.0 based it is likely to be able to handle older processors fine. The X570 is unnecessarily restrictive. MSI on Twitter actually agreed when I deemed a new X570 motherboard as useless due to its inability to use my R5 2400G processor.
I have heard some A320 boards have updated BIOS to support a CPU upgrade. Check for specifics.
Most likely stores will have old processors for at least a few months and eventually some discounting will be seen as the last few are sold off to make way for new products.
Ryzen 3000 series processors will be available for some time until TSMC can get their 5nm line producing processors with an acceptable yield. AMD will use the 7nm line for the Ryzen 4000 series which have already been taped out. These should be widely available by summer 2020.
Looking further down the road, the JEDEC working group is still a fair ways off before DDR5 will be ready. Servers will be first in line and consumer hardware will follow typically 8-12 months down the road. When AMD moves to DDR5 most likely they will also adopt PCIe 5.0 which is already ratified.