I always have a couple of notebooks handy, and a few mechanical pencils and fountain pens.  The drawings you see here were generally done outdoors, or from sitting around in cafés.  Sitting around in cafés is actually one of my favourite kinds of research.  I use pen, but also brush and ink.  The busker far left is with pen, while the young lady near left was done in brush.  I use the nifty Pentel brush ink pen - reliable and always good fun.  (To a get a good idea of the difference between pen and brush inking, take a close look and compare the pages of Tintin with Asterix.)

The notebooks I use are usually  A5 or A4 - but then sometimes you should try another format just to break up your rhythm and see things differently  Recently I've discovered that in second hand or Church Mission stores they sell a lot of photo albums, which tend to be completely unused and have really good paper - deep and lush. Try one; they're fun to work with, and it'll cost you a fraction of an Art Pad.   The guy far left was done with pen on tinted paper, the two lovebirds  with a dip pen, brush and wash.

The two guys above were drawn in ink on tinted paper, the guy far left in a dip pen, and the buildings with a fountain pen.  I was given a Cross fountain pen as a gift - very reliable, a pen that will last a lifetime - I totally recommend them.

Technical pencil above, the other two pen and ink.