90 lb. paper is available in all hobby/art shops, but I would not recommend this weight. It is very difficult for a beginner to work with since it buckles so easily.
Watercolor paper can be purchased in large sheets as well; however, it is more expensive. I would not recommend this type of paper for the beginner as I have seen beginner watercolor artists nervous about using the more expensive paper; therefore, they are tentative and will not allow themselves to enjoy the experience of painting. As confidence is developed, more expensive paper can be used.
I usually purchase 11" x 15" paper. This size can be cut down for smaller paintings. An 11" x 15" painting is a good size for a beginner. A pack of 12 sheets of 140 lb. cold press paper usually sells for less than $8.