It is probably easier to discuss, first of all, which pattern cutting paper you should not use:
You can buy white sheets of pattern paper - they have the consistency of the paper you get with commercial patterns - in other words quite thin. This is not the kind of paper which is appropriate - you need something a little thicker.
Dot & Cross Paper - it has the right thickness, but also has, as the name says, markings on it. These dots and crosses might be quite confusing when you try to develop a style.
Brown Wrapping Paper - often comes in a roll to wrap parcels ready to be sent off. This paper is not bad, a little thick, but could be used as an initial alternative, if you cannot get any proper pattern cutting paper, as it is widely available. Big disadvantage: it is not transparent. You will often need transparency, if you need to copy something to another sheet of paper.
Plain pattern cutting paper, which has the same thickness as dot & cross paper. It has the right thickness and is transparent.
Where can I get professional pattern cutting paper?
This question is easy to answer for those who live in the United Kingdom - there is an established company in London which has specialist pattern cutting equipment and paper.
If you are serious about pattern cutting, you can order a roll from them - choose the Plain Tracing Paper (91 cm x 300m).
If you live in another country - there are suppliers out there - use the term "pattern cutting equipment", "pattern paper", "pattern cutting paper" and other related terms - you should get a result!