Pecatonica River is another good one. I've bought stuff from them also.
Building your own BP rifle is great fun! As best I remember I spent about 250 hrs and $400 on my first one. Of course a lot of that time was making jigs and such that I didn't have on hand. Now I do!

Just understand, with modern rifles everything is built around the action. With ML's, everything is attached to the stock. Don't worry about your woodworking skills, you'll suprise yourself!
When it comes to rifle design, I felt like you the first time. Then as I went along I drifted into a desire to recreate the works of the old masters. Got a Lancaster in the works now.
Then one day, while sitting in a tree waiting for Bambi, I designed the perfect hunting rifle in my mind. With a few exceptions, what I envisioned turned out to look like a Leman Trade Rifle! So I got a set of full sized plans for the Leman percussion half stock.

If I ever build that rifle I'll use those plans except for a hooked breach and a fast twist barrel for 54 caliber Great Plains conicals!

Look up the Leman on the TotW and Pecatonica sites.
Whatever you do you'll have a grand time!