Plan B or the morning after pill will not end a pregnancy they are used to prevent a pregnancy. It will not abort a fetus.
Don't panic. You aren't the first person to accidentally get pregnant a second time. It is your body and you are free to do with it as you may. The cheapest and least invasive way would be the plan B pill which you can get from your local pharmacy or planned parenthood, without a prescription. The issue with that is it is called the morning after pill for a reason. You should consult a pharmacist or a physician as to how long after you become pregnant that you can take this pill. If it has been too long, unfortunately you will be subject to another conventional abortion procedure. After this is all over with please remember the old saying, "An ounce of prevention is worth a pound of cure." Please also keep in mind, these procedures are very traumatic and 99% preventable. If you boyfriend loves you or even just cares about you as a person, he won't even touch you without a condom on. I am not a doctor or a health professional, and I think you should definitely seek advisement from one or both. Good luck. Others, please try to be objective and put your beliefs aside, this is a medical site to help people. If you can't be helpful, simply don't respond.
There are no doctors on this forum. If you want a second abortion, I would assume you could get it. Have you considered perhaps adoption though? Or perhaps keeping the child? It might seem impossible, but nothing truly is and life doesn't always play out the way we want it to. As for the best way, there is no "best" way as they're all horrible methods. You also need to look into birth control AND using condoms AT ALL TIMES. Abortion should NEVER be used as BC.