Weighted vest advice for a newbie?

[JASPER]

I train in martial arts, and have been thinking about increasing the potential of my workout with a weighted vest. Is this a good idea? I study Tae Kwon Do.

Even if it's not, I still want to use one just for regular exercise. However, I've never used one before, and they come in different weights i.e 20 or 40, so are there any suggestions for a good starting weight?

And at last, does anyone know where to order a decent vest from a reliable website?

Thank you.