Where can I buy something that will allow me to plug a mic/line in into my Mac Notebook? (reasonably priced)?

Answer 1

You basically have to have a soundcard that supports an external mic. It would have regular small jack plug like you would use for headphones. If it does'nt then it is not going to be cheap to have one installed.

Answer 2

In the future, it helps to know more about your computer specs when asking tech questions. But here's a USB adapter you can get:
http://www.griffintechnology.com/products/imic/

Also, pretty much any off-the-shelf USB microphone will work with OS X. This is cheaper, because with the adapter option, you still have to get a Mic.