Beyond the core gameplay changes, the Anniversary Edition features two entirely new sets of levels that weren't in the original GBA release. These are arguably the best reasons to seek out the ROM:
For those who have a modded Nintendo 3DS, is the definitive source. It is a custom eShop replacement that hosts a massive library of delisted DSiWare and 3DS titles, including the Anniversary Edition. The files are in the .cia format, ready to be installed directly onto a 3DS's home screen.
Here are the most reliable places where the ROM is known to be hosted: