Newbie with a roof leak. Please Help!

[JackP]

I just purchased a 72 overlander and have a roof leak where the antenna crank enters the roof of the trailer. I want to remove the antenna and patch the hole but I have a few questions.

Where do I get the sheet aluminum to make the patch and what thickness do I get?

From what I read here, Vulkem seems to be the sealant of choice. Can I purchase that locally and exactly which Vulkem sealant do I get?

Lastly when it comes to rivets...which rivets do I use for this repair? It seems that with some rivets the post (the nail like thing) in the middle stays inside the installed rivet and others don't. (Am i correct about this?)

I know these are very basic questions and I do appreciated any help.
Jack