What sort of budget do you have? A pen input is a specialised thing - You need a laptop that supports it out of the box, you can't just get a pen that will work. You will see cheap touch screen stylus things online but they aren't suitable for writing accurately - A laptop with a proper pen input is very similar to those Wacom graphics tablets where the pen is tracked extremely accurately and with pressure sensitivity.
Off the top of my head I can think of the Lenovo ThinkPad Yoga which provides pen input - It's a great machine but is also quite expensive. You can probably get it used for a reasonable price. There are also older laptops such as the ThinkPad x230t and x220t (note the "t" on the end of the model) which can be picked up very cheaply used/refurbished, although they are a bit more bulky than a modern convertible machine.
