Hi
You can go to most electrical/hardware stores and buy a universal pack.
Basically you use the old filters as templates and cut to size.
As for changing, how long is a piecs of string 😕
Rule of thumb would be 6- 12 months depending on use. Tell tale signs are visual (greased up) and odour (increase of smell in room from cooking) and poor extraction (which s the same if the metal grease filters are clogged also)
Thats the charcoal filters acknowledged, the metal grease filters usually can be dishwashed (check first)