If you want truly heavy, substantial doors, that have enough mass to blot out noise, look at FD30 fire doors.
If you are going to varnish them, you can get veneered ones; if not, they usually have a pressed smooth surface for painting (fake woodgrain is available if you like naff).
They are 44mm thick, not 35mm like flimsy modern doors. They are best hung on lift-off hinges. Due to the thickness you will need to move the stop on the lining ("frame") or fit a matching lining. 44mm is the thickness of an ordinary front door.
They are available in numerous sizes and designs. I like the 6-panel ones because I grew up in an Edwardian house.
Solid core are usually 35mm thick so will fit into a standard frame and are less heavy than FDs.
For knobs and handles, look at IronmongeryDirect who have a vast range. If you want knobs, you need a longer backset or you will catch your knuckles on the doorframe. You can get modern tubular mortice latches up to 150mm fairly easily, or horizontal mortice sashlocks if you have a real or replica old house. If you have a real old house the door linings and original doors may have been made to suit a thick door. If you are used to thick heavy doors you will be unimpressed by anything less.
Premdor is another large maker.
I have seen solid Maple firedoors in a prestige building and have forgotten the price, but it was colossal.