i agree they are out of stock at the moment, i bought them in Feb, only 4 months ago. what i said is do a bit of digging, there's compatible ones out there and ways around the firmware. Buy from amazon, if the supplier says compatible, you can send them back, i had to send a officeworld set back because of this. if you want to save money, just do a bit of investigation
The cartridges (HP 912) for these printers are not available yet in either Remanufactured or Compatible form. The range itself has been out the best part of a year, so don't expect cheap cartridges anytime soon. Regarding the HP 7720 mentioned above by @smithers1874 - Updates happen every few months and manufacturers have to reprogramme chips every time there's a change... Current working version is v9.
Printers either cost bazillions now or are rare as hen's teeth.
there is indeed a way to get round it. you could disable the firmware updates on your printer but i didn't so did this and this worked https://www.bestprice.ink/en/support/how-to-disable-hp-cartridge-protection-on-my-printer-24 have a 7720 and first set of cartridges didn't work. i then went to these guys and they work fine on my HP https://www.amazon.co.uk/gp/product/B07YX3JDV6/ref=ppx_yo_dt_b_asin_title_o03_s00?ie=UTF8&psc=1 just do a bit of digging
HP also push out firmware to their printers that block non-HP ink. No way to disable. Look up "dynamic security feature"