They have been hard to find ever since the combination of the following:
(1) The 2011 Japanese earthquake/tsunami which wrecked production for quite a while.
(2) The advent of the Sawyer Squeeze, when it was found that Platypus bottles wouldn't work to replace the flimsy Sawyer bags.
Ever since, demand has greatly exceeded supply. Shipments that come in are sold out within days. Evernew still hasn't been able to catch up with the demand.
Luckily, I switched from Platypus to Evernew before the above events, because I was continually losing the Platypus caps. I therefore have enough.
About all you can do is to check the various suppliers daily and order immediately when a supplier has them in stock. I don't know if any firms will allow you to put in a backorder request to get in line for the next shipment, but that would be ideal.