Patternmaking Paper can refer to both the heavy-duty cardboard (around 225gsm) used to make blocks, and thinner paper used to make the patterns which are pinned onto the fabric.
Patternmaking suppliers sell a variety of papers including special Dot/Cross Marking Paper which is a highquality option; it is partially see-through, and the marks on it help you to keep your lines and angles correct. It is also quite expensive.
Blocks | Slopers
For Blocks and Slopers, you need the heavier cardboard. Buying the heavier cardboard in large enough sheets to make your Pants and Extended Line Dresses Blocks can be a challenge for the home sewer. If you live in a city near a TAFE that offers Patternmaking subjects, you are in luck. It is also very easy to buy online from places such as EM Greenfields, but the postage will mean that it won't end up cheap. The Patternmaking cardboard comes in sheets of 120 x 74 cm, so if you buy 10 or more sheets it may be worth the postage. You can buy the Dot/Cross paper from these suppliers also.
Paper for the Pattern
For the pattern that will be pinned onto the fabric, you can use a variety of different papers.
This second image shows the dot/cross patternmaking paper that is one option used by dressmakers. It is sold in rolls of 100m+ by wholesalers such as M Recht.
You can also buy pattern tracing paper from your local sewing shop, but this is also very expensive. I find brown kraft paper works perfectly alright for heavier fabrics, including most cottons.
For finer fabric, the kraft paper is too thick and you need to look at other options such as tissue paper (if you can get the right thickness and in big enough sheets) and another alternative is using non-woven non-fusible interfacing (vilene).
I buy Patternmaking Cardboard online from EM Greenfields in Sydney and have it shipped (it's actually called Pattern Paper). I us brown Kraft paper for patterns when making garments out of thicker fabric, and for finer fabrics I use vilene. (I do not receive any commission or payment from EM Greenfields).