Here is a tip for people who like the idea of using watercolour blocks, but don't like the price of the ones that you can buy: it's not difficult to make your own.
You will need 10 to 15 sheets of watercolour paper cut to a little larger than the size you want the finished block to be; a similar-sized piece of mountboard or stiff card (it should be acid-free if possible); two more similar-sized pieces of thick card of any type; PVA adhesive; a sharp craft knife and a heavy cutting guide.
Make the watercolour paper into a stack with the piece of mountboard at the bottom, and mark out on the top sheet the exact size of the block.
Cut down through all of the sheets and the card to leave a clean edge. This needs to be done carefully as it's easy to let the angle of the knife drift and end up with a bevelled edge. It won't affect the function of the block, but it doesn't look so good. The best way is to use a light pressure and gradually cut through. From this point try not to disturb any of the sheets in the block, so that you keep a perfect edge.
Next, cut two pieces of card slightly smaller than your block. These will go on either side of your block while you are glueing it. One is to raise the block off the work surface, the other is so that you can weigh it down evenly on the top.
Assemble all of the components on your work surface, so that you have a piece of card, followed by the block of paper, then the other piece of card, and weigh the whole of it down with a heavy weight or several heavy books (I have used a piece of MDF as well, just to make sure that the weight is distributed evenly).
On one side of the block, mark two vertical lines about 4 to 5 cm apart. These will be a reminder to leave a gap without adhesive, so that it will be easier to remove sheets after they have been painted on. Then apply adhesive to all four sides of the block, spreading it out evenly with a smooth stick or knife. It doesn't need to be very thick, but make sure to cover all of the edges of the paper sheets and the bottom piece of card. Leave it overnight and the next day you will have a block of watercolour paper ready to use.
The final thing, for convenience, is to attach a sheet of paper of your choice to the back top edge of the block, so that it can fold over and protect the front when not in use. When you have finished a painting, insert a clean knife into the unglued portion of the edge and slide it around the block to detach the sheet.
The nice thing about home-made blocks is that you can make them any size and shape that you like, so that if you've always fancied a long, landscape format block, or a square one, that's no problem.
If you haven't used blocks before, you may be disappointed to find that the paper still cockles sometimes. This happens even with manufactured blocks and is quite normal. The paper will go flat again when it dries. It is a mistake to think that paper in blocks behaves in the same way as paper that is stretched on a board in the studio.
One final point: if the edges come apart with wear (I have had this happen even on good quality Arches blocks), just weigh down the block again and apply more adhesive.
If you have any questions, please leave a comment, or send an email, and I will do my best to answer them. I would also be interested to hear any suggestions for improving the process.
If you are making blocks on a regular basis, René's idea (see comment below) is probably easier. Cut two pieces of MDF, plywood or very stiff card slightly smaller than your block, and then use clamps to sandwich the block between them.
Sue Johnson has an interesting idea for making sketchbooks here, and a nice example of a printed cover for a block.