I'm possibly going to get an internship at a company this summer, and since its a unpaid internship, I have no real way of paying for a bus pass to get to work every day.
Is there a way to politely bring up the subject?
The best thing to do is just ask. If there are other interns there ask around and see if anyone else has the same issue of needing to get a bus pass from the company. If there are no others that you can find, ask to meet with the boss in an informal meeting and just bring up that you wanted to know if there were any ways for the company to help you out by getting a bus pass. Just be careful about doing this and make sure that having transportation was not part of the requirements for the job.
Also if you do get a "no" from them. Drop the question completely. pushing for something like this after a no was already given could create tension between you and your boss and if the internship has a chance of being turned into a full time job, it could put a negative mark on your notes.
A lot of companies have bus programs where you can get your pass through your employer. This is usually taken out of your pretax pay, but since this is an unpaid internship I'm not sure if you could get something worked out. Companies like this because they get tax credit for subsidizing public transit. Paying for your bus is good for your company and you, win-win!
Do some research into your city/state's public transit through work program, and send them some links either after you talk, or in the email you ask about a bus pass in. The more you can do for them on this the better, and that includes the research.
The main aim of anyone who is working is to earn money. Since you are working as an intern without pay, it must be difficllt you to pay for your conveyance. Avoid sending an e-mail or indirect approaches. Take an appointment and have the conversation across the table with your boss. At the same time, don't forget your company rules. Each company and offices have their own norms and rules. Perform better initially and get noticed and recognized for quality work so that when you discuss about your pay rise, you have genuine reasons and clear points to consider.