What kind of money are you looking to spend? Do you want flintlock or percussion? Something full sized, or lightweight and short pull?
Without going to a custom made gun, there are very few choices in .36cal available nowadays. (sometimes there are used ones on auction sites and some gun shops)
Deer Creek carries the CVA Squirrel Rifle, H&A Underhammer, Traditions Shenandoah, and the Mowrey. That is probably your best selection (in one place) for new .36cal guns.
Deer Creek's phone number is 1-765-525-6181 They do not have a website, but are very helpful and have a catalog available.
A nice "used" .36cal guns would be the sadly discontinued T/C Cherokee or T/C Seneca. BUT, they can bring prices as high as $400 on the auction sites.
Dixie Gun Works carries some small bore rifles too. They do have a website with prices and descriptions listed.
Good Luck!
WV_Hillbilly