I just replaced this o-ring on my classic pro. As far as I can tell, most seal replacement kits or individual parts on Wholelattelove/Amazon/etc are for the pre 2019 version, and don't fit the classic pro. I measured it, and it's a metric size, 8 mm inner diameter x 2 mm cross section. A little tricky to find at a hardware store in the states, but Amaxon gas them:

https://smile.amazon.com/dp/B07CVJ2WZS/ref=cm_sw_r_cp_apa_fab_mjtAFb29C12FA