http://www.middlesexvillagetrading.com/

http://www.trackofthewolf.com/index.aspx

http://www.muzzleloadingforum.com/Benefits.html

If you are wanting to stay within the 18th century, then the T/C Hawken would not be your choice. Although I would think that a T/C Hawken would likely be your best first ML.

For $400 or less, you won't likely find anything that is period correct to the 18th Century. More like $800 on up to several thousand.

A pretty nice 18th early 19th century-ish rifle is the Pedersoli Frontier/Cabela's Blue Ridge.
I've got the Frontier in 50cal and it's a really nice long rifle.
The Cabela's Blue Ridge would fall near the $4-500 arena.
I bought the caplock version and also a drop-in flintlock conversion.

i would recommend looking at lyman blackpowder rifles.