Using craigslist in the case works very well and in most cases free. Wayup.com has a pricing model that is really not favorable to us since you pay per applciation and per filter. That can really start to add up unless there are SPECIFIC skills you are looking for in an intern. Even at that rate chances are you are going to burn through a few hundred dollars before finding someone.
Beyond CL - I think you should find the job board / careers page of the local university from which you would like to pull candidates and post on there. Many of those are free and are quite frequented by college students.
One thing I always do is be completely transparent about the pay, the role and the future opportunity with the company. That usually attracts more people. If it is possible to allow them to work remote after a certain time - include that as that will drive more applications. We have a lot of interns that moved to remote after they have been acclimated to the work environment and the position.
Hope this helps!