Indian Ink is found in most crafts and all art stores - it will come as a liquid in a small bottle. You will also need to purchase a handle with various pen nibs (tips) if you want to write, although a fine paint brush could work if you are patient. You can varnish or spray a clear coat over it with not problem.

Plan to practice with it a little first to get the hang of dipping in the pen nib to collect the ink and then writing. While it appears to be a very watery liquid, it is very concentrated dye, so protect any surface that you don't want stained. It is a little like the old fashioned fountain pen that you dipped into ink wells in order to write.

You can spray polyurethane over a Sharpie marker or paint pen by doing it like this: (this knowledge comes from many, MANY bad experiences)

Let the writing dry completely.
Holding the object to be sprayed at arm's length, spray with a quick, light pass. Let dry.
Do it again. Let dry.
This light spray 'sets' the paint under tiny drops of poly.
Spray again a little slower. Let dry.
Paint should now be nicely protected under enough poly that you can spray normally to build a good coat and shine.

I've had really good luck using an Identi-Pen by Sakura. I found that Sharpies sometimes fade over a couple of years.

Let dry completely and spray as Dian says.

I agree with aggiewife - the Sharpies absolutely do fade - which is why artists use India Ink for a professional finish - it won't fade.

Or you could use a paint marker or I have even dipped a pen in paint and used that in the past. It takes a lot of dipping but it got the job done and then after years of practice, practice, practice now, I can use a really small brush and paint the words on by hand :)