Where can you get the Rustoleum spray paint? Heck...I got mine at Meijer. You could probably find it at Wal-Mart, KMart, any store that has a lot of spray paint.
More durable then the stock paintjob? Mmmmm...I dunno about that. I've got little scratches here and there, but it hasn't been too bad overall. Probably not as durable as something like Renegade mentioned.
Here's the pic of my painted Tippy (by the way, I don't have the Flatline on my marker anymore. I went back to a J&J):
"If you don't stand for something, you'll fall for anything."