Here's how I'd approach it. You're buying a backup / deep concealment gun for personal protection. It's not for bragging rights or showing off at the range. Therefore a certain cold-bloodedness is needed when deciding what to buy.
(1) Is 9mm effective as a defensive round? By all accounts, the answer to this one appears to be 'yes'. Do more research if you want, but modern 9mm JHP appears to be effective as a defensive round in a handgun.
(2) You already have a G19 9mm handgun. Glock magazines are interchangeable, so you can use your G19 magazines in the G26.
(3) Money appears to be a consideration for you, because otherwise you'd just buy whatever you wanted without thinking long and hard about it.
With that in mind, I'd set the G26.
Here's why: you already have a 9mm carry piece. The .40 is not replacing it, it's a backup. Having one caliber for carrying purposes means that when you go buy ammunition, you can go to town and replenish your stockpile of 9mm in bulk and never look back. Since you already have a Glock, you can buy a bunch of G17, G18, and G19 'standard capacity' magazines and use them in either of your guns.
The .40, .45, 10mm, etc. all have their good points, and if you were starting from scratch, maybe that would be enough to justify going with one of those. But if this is for serious use and not a range toy, and price is a factor, there's no good reason to switch. Plenty of people have saved their behinds with 'only' a 9mm. And, again, you already have one.
Regarding availability of ammunition, I do not see a shortage right now at all. Plenty of 9mm ammo is available for sale online if your local stores do not have it, and reasonable (ish) prices. It certainly is less expensive than any of the other calibers suggested.