Where is a good place to buy WW1 RAF decals for a Sopwith Camel I am building?

Callie Graphics may have what you need:

https://callie-graphics.com/collections/sopwith-camel

Callie Graphics is da best

+1 on Callie. She has a variety of scales "on the shelf" but if you need something in an odd size she can accommodate your needs. If you have a good picture she can also do custom markings.

Thank you so much! Do you know what scale a 1.2m wingspan Sopwith camel would be? 1/7?

Thank you so much

It would be 1/7.1 so close enough. At 1/7 scale the wing chord should be 178mm or right at 7 inches. On most camels the roundels stretch from leading to trailing edge so you will want a diameter of at least 6 3/4 inches of 160 mm. Since I dont know what kit or plans you are using I would confirm all of those measurements.

They are just circles, paint them.