Cool, thanks for the feedback, guys. I have been checking craigslist like crazy, but I don't know how to build a bike. Also, I am currently struggling with trying to find a job, which is why I would prefer something affordable. I also do not mind whether or not it is used, I just want it to look nice and serve its purpose.
A BMX bike would not work for me.
I need physical training because I'm going to be entering the police academy in a few months. I just want to gain more stamina and such, and I strongly believe that a bike would be an amazing tool for that.
I will check all those links and sizes.
Thanks again for the feedback.