You may get from the online site “pupsik” and search for “maymom” brand which is compatible to use for Spectra pump.
Spectra do not have a 20mm flange, would need to get from third party manufacturers like maymom
I got maymom flange from shopee instead they have smaller size and suitable for spectra too.
Kiddy palace or mothercare
Maymom flange from Qoo10
got mine from lazada
Shoppee has too
Pupsik or qoo10
Kiddy palace
Online store
Shan