Is there any way to buy an extra memory card and put it in my phone?

It is not possible to add storage space to an iPhone. If you need extra space, you will need to move or delete existing content from the phone.

Yes, you could delete some apps, but as apps are usually quite small, removing them would make very little difference to the available space. Files such as video, photos, or music would use far more space. If it was possible to move those files to a computer, it would free up more space.