The retaining ring can be found at most any Mustang vendor, (I'm looking at it now in the California Mustang catalogue for $8.16). I can't tell what the other part you have circled is.
Yes. Not visible if the fender and bucket fit together correctly. I do not know if they can be purchased separately or not but if not put a bead of silicone on there and let it harden, then attach the bucket and don't worry about it.