Antec and Ultra both make inexpensive case fans. For a few dollars more they include a 3 way speed switch and LED colors of your choice. I go to Fry's Electronics, but you can shop by mail for fans also.
I'd go with some Yate Loon fans. Very cheap, and good quality too, we had some discussion on them in this link: https://www.techspot.com/vb/topic90065.html
In that link there are some links to reviews of them.