These will be readily available from your local Jaycar or similar electrical supply store. I don't normally supply these as they triple the shipping weight and the power supply is normally specific to the country. BUT, you cannot execute code from flash and write to it at the same time. I have done this by copying a flash write routine to RAM, then running code from RAM while the flash writes. We normally only use 1Mbit chips due to cost and availability.
All flash is expensive at the moment and in short supply.
It now costs about 4x what it did 2 years ago. We rework a few boards, exchanging the flash chips when only one or two are required We can fit in with a production cycle where we will have to be do them in multiples of 12. The additional cost will depend on the availability and cost price of the larger flash chip at the time. Normal use would greatly extend this number. I haven't had one fail or had a failure reported yet.