The Holmes HAPF21 (G) replacement air filter and the HAP222 itself has been discontinued, but if you still want to keep using it you can buy a Honeywell (or generic Flintar) HRF201B (“U”) filter and cut it to size with scissors. It’s almost exactly the same width and just a little bit longer, and as long as there’s no substantial air leakage it’ll still do a great job of filtering. The carbon pre-filters seem like they’re still somewhat available, but in any case they’re generic too and can also be cut to size.

The only downside is that it’s about 1/4″ too tall for the unit so the lid doesn’t fit tightly anymore, but that often doesn’t matter to me anyway.