Don't know about aftermarket manuals, I've had the HD manuals and liked them.
I've had good luck with these folks especially if you buy more than several items at a time. Save on shipping that way.
You will either need the part number or call them for it if you can't find it on the Harley website.
Good luck, sounds like a good project. If you need specific advice there are only about 252,678 posts regarding Sportster mods here.