Over the past few days, many of my ebooks have been showing on Amazon.com with a warning saying 'This title is not currently available for purchase'

There are two ways to get around this issue:
First, you can buy from your nearest Amazon site. (E.g. those in Australia or NZ will need to buy from amazon.com.au)

The second method, which is the one suggested by Amazon Customer Support, is to temporarily add Amazon's US head office address to your Kindle account.

The latter fixed it for me!