Do they sell flags onsite.

Yes

Yes sir, they sell two different sizes. One is longer so it is able to be used at other parks that require longer flags like Silver lake sand dunes. But silver lake doesn't allow a mesh flag so make sure if you get a longer one for that reason that it has a solid flag and not mesh.

Yes they do. $20-$35 for flags and they sell mount brackets also

yes, starting at $20