That looks to be a pretty generic microfilter. You can get it in sheets at the supermarket where I live, and you just cut it to size. Here is one at the hardware store.
Many items to choose from, this is just an example. Listings called "HQRP Pre-motor & Air Clean Filter Kit for Miele Vacuum Cleaners, 905 SF-SAC 20" also might work, you'd only need the one square pad from it.
Attachments
UNIVERSAL VACUUM CLEANER PRE MOTOR FILTER CUT TO SIZE For Miele Electrolux Volta - vic aus.jpg