Bienvenu au nation, Phillippe! You will find lots of helpful folks and information here on all things CBG (CBU, CBB, etc.) Feel free to poke around, ask questions, and post your stuff (photos, recordings, videos, etc.).
I assume you mean the sound hole/resonator cover for a Dobro style guitar.
Well the only place you are likely to find one is on ebay. If your search comes up with nothing try ebay.com were there have been several listed of late, but they are expensive.
Why not try making one from something you have lying about.
Also have a look at bairfoot cajun's page on the nation, and go thro his photos, as he makes a lot of resonators, and the covers. He is very helpful so you can send him a message and he will tell you how he makes them.
Don't be afraid to go over the photo section and, if you see something that you like just message them and they will help as well.
They are real friendly people on here and will usually give you the information you need.
Hope this helps.