Have you ever beaten Paper Mario, Paper Mario? I have. And the scene where the Bullet Bills are at Koopa Bros. Fortress happens only once... There are some more Golden Bullet Bills at Bowser's Castle (they're called Bombshell Bills).
By the way... Bowser's Castle in Paper Mario is the best castle of any Mario game! It completely stands for what Bowser's Castle SHOULD be like: lot's of traps, lava, Bullet Bills, Magikoopas, Dry Bones, puzzles, Boney Beetles (the enemies from Super Mario World). Bowser's Castle is by far the coolest level in the entire Paper Mario game!
------------------------------
"Bwa ha ha! Guess who it is, loyal Mario fans...
It is I, King Bowser... ruler of all Koopas, big and small. I have come to take over the Mushroom Kingdom (no, not the place...I mean the WEB SITE!)! Deezer, you will surrender your troops to me, or else you will never see your precious Mario again! Bwa ha ha ha ha ha ha ha ha ha!"