First and foremost, check out last week’s post on addons that are ready for 5.0.4. I’ve been doing a ton of updating on this piece every time someone informs me that an addon is updated. It’s a work in progress, of course, but it’s pretty accurate right now. If you don’t see your preferred addon on there, then have a look on the big three addon sites: WoWInterface, WoWAce and Curseforge. You’re looking for recently updated files in the download section.
If they don’t show up there, have a Google. Your addon may have a stand-alone site. You can look elsewhere, but these are the places where the vast majority of addons are hosted with the authors’ permission, meaning they are in line with copyright rules, ethically sound and home to the authors’ most recent updates.
Once you’ve checked for the most recent versions, download any that show compatibility with 5.0.4. If there are none, then make a note of that addon and check back in a few days’ time. WoWInterface can also be set up to email you when addons are updated; check out how to set up email notification for addon updates.