I'm somewhat new to muzzleloading, and I've been looking around trying to find reproduction rifles and other supplies.

what are some of the more popular online retailers for muskets, black powder,  and others supplies?

anything helps

track of the wolf for supplies
Midsouth Shooters Supply

Agreed, and add muzzle-loaders.com out of Oregon to the list as well.

Track of the Wolf

Track of the Wolf is the best to be honest. Great selection, customer service and shipping is oddly fast.  They have everything. Dixie Gunworks, and RMC OxYoke  and Ocotber Country Muzzleloading are pretty good to. Idk much about inline stuff so I can't help you there.

I have had good luck with Tennessee Valley Muzzleloading, and Track of the Wolf as said before.  Also I like to cruise Claysmithguns.com once in a while.  He has interesting stuff that rolls through the "available guns' part.

Townsends.us has a lot of interesting period stuff, and really nice folks to deal with.  Turkeyfoot trading company has some interesting stuff, along with CrazyCrow.com.

Danny Caywood at caywoodguns.com is a great guy, but has been having pretty major health issues.

If you feel like building your own, Jim Kibler at Kiblerslongrifles.com builds probably the most finished kit out there that is getting wonderful reviews.

Hope that helps!!

Doc