Also use Seawhite (in the form of sketchbooks)
For pen and wash stuff I buy basic-grade +300gsm watercolour paper online
Then put 15-20 same-size sheets in a pile on top a piece of backing-card.
Then hold all together with four to six bulldog clips.
Then paint all four edges with Copydex glue.
It makes a decent sketching/painting block and stops the paper cockling when doing washes. Always leave a little 1cm gap in the glue so you can detach the top sheet by inserting a palette knife and skimming around.