What model is your Macbook Pro? If it is one with a user replaceable hard drive,
just get a bigger drive.
Also, if your library is that large, any cloud storage will be expensive and of course will
be subject to availability (of course the internet is always, always available as well as cloud
servers).
Best bet is to make a local copy. If you have a Macbook Pro, depending on model, you
could use an SD Card for your iTunes library which will make it pretty unobtrusive compared
to the external storage or you could get a USB Flash drive (easier than a cable and external
disk drive), that is assuming that you can get a device large enough for your iTunes library.
Another suggestion, you may want to do some major house cleaning on your computer.
If you have photos or documents or such that you haven't looked at for a long time, archive
them on an external device.