If you had white, he may appear in white forest, you have to be lucky though since unless you have black to keep getting more people to go to white forest, you may not be able to get him.
Or if you have dream world, bagon is at the rugged mountain when you have over 10,000 dream points
Ether transfer it from 3rd or 4th generation games of get lucky in the white forest in white version and find a bagon here's a link to a site with the info.
