I am playing Storm King’s Thunder as a tortle. I was wondering if I could pay to have armor specially smelted to fit my shell. This is not considering the cost or time taken to make it. Would it be possible?
According to The Tortle Package
Due to your shell and the shape of your body, you are ill-suited to wearing armor. Your shell provides ample protection, however; it gives you a base AC of 17 (your Dexterity modifier doesn't affect this number). You gain no benefit from wearing armor, but if you are using a shield, you can apply the shield's bonus as normal.
While it says that you are ill-suited to armor, it specifically states that you gain no advantage from wearing armor. NOT that you are unable to wear armor. It also explains that your shell gives you a base AC of 17 (The only non-magical armor that is higher is full Plate armor at 1,500gp and AC 18).
So RAW, you would not gain any advantage from putting any armor on, regardless of how well fit it is. There are many other ways to increase your AC. And if your DM thinks that having extra armor custom made around your shell makes sense, of course, they may decide otherwise.