They're listed all the time - the Flex and Instants i bought very recently. I just ordered a Flex three days ago.

Check daily, usually first thing morning. https://old.reddit.com/r/UbiquitiInStock/ is handy, but i've often found stock when it wasn't posted - so i think my morning check is a bit better than the stock posts.

edit: to be clear, i've bought all of my stuff (within the last year) from Ubiquiti directly. No 3rd party inflated prices