At your local Briggs dealer.
Maybe you can order on-line at Nikki
Or just measure the ring and find an equivalent at your local hardware store, Viton or NBR should do fine.
F-250 5.4 V8 stand. cab, tandem trailer
Ferris IS3100Z 61" L Kaw, Hydrocut 36" 13hp Kaw
Shindaiwa T3410, EB8520 Maruyama HT2300, MS047
Stihl FC85, MS192T, 041AV, MS200