what i did with my harry potter scarf is that i used Red Heart super saver yarn, its a worsted weight and in one skein you get 364 yards for about $2.17. I would recommend size 8 needles with this yarn.
here is the pattern that i used in case you dont have one. you can choose which style you want.
http://knit.atypically.net/scarves/azkaban/pattern.shtml
http://knit.atypically.net/scarves/hogwarts/pattern.shtml
Try Lion Brand Yarn, its usually reasonably priced.
They have one called Wool-Ease. Its like wool, but doesn't feel like the itchy wool every one knows. This type you can use size 8(US) knitting needles. When you go to the craft store, just feel the skein. If it feels itchy to you, don't buy it. If you have a local yarn/needlework store they may be able to recommend a yarn for this type of project.
Hope this helps, happy knitting!
lion brand yarn is the best my mum uses it all the time !!!
im not a big knitter, but you should use some lion brand yarn. my grandma uses it and it is really nice :)