I have the KIPer bag from knitpicks.com. I really like it. I have the large one (as I wanted to be able to put my work stuff and my knitting (usually sweaters) in the same bag. I actually find it slightly too large and am considering buying the medium size one for other days. Although the large one made an EXCELLENT carry on bag for the plane. Room for personal items, snacks, mags and knitting!
I also have the knitpicks options needles which come in a little zipper book like a daytimer. That is very handy.
I think any bag that you like will work as a knitting bag, unless you want a bag that has specific pockets for needles, etc. I think Target has an excellent assortment of bags that cost about $20. I also saw a lot of really cute business satchels in Staples the other day that would make great knitting bags.