I always imagined that many good dragons would spend a fair amount of time in human/elven/half-elven form...joining adventuring bands, subtley affecting action from behind the scences, etc.
As others have noted:
(1) From evil dragons and/or other foul beasts
(2) From misguided or evil dragon-slayers
(3) Depending on the campaign, he/she could set-up a viable commercial enterprise - Dragonwing Express
! OK...sounds corny, but a dragon could make a mint transporting goods or people from one place to another, especially while they are younger.
(4) Tribute and/or gifts from those the dragon helps/assists.
I would also imagine that much of the dragon's monetary wealth would be converted into things aesthetically pleasing to the dragon, instead of laying about in an unkempt horde. The more avaricious of the metallic goods (copper/bronze) might very well invest their starting wealth in various commercial enterprises...seeking to become vastly wealthy.
In fact, horde gathering for this type of dragon might well be a "game", with friendly competition between dragons to see who can accumulate the most prodigious horde and/or who can use their wealth to throw the best parties. Commercial enterprises of such wyrms would be fairly safe from bandit and intelligent monster activity, once the dragon's trade mark is known.
Bandit leader,
"That caravan bears the mark of wyrm X...leave it be or we will all reside in it's stomach! Let's wait for easier prey..."
~ Old One