can i buy more disk space for my mac?

A strong maybe. A lot depends on what Mac you have.

If it is a Mac with a DVD drive, you could replace the DVD with another disk or an SSD.

If it is a Mac with a rotating hard disk, you can replace the disk with a larger disk.

If it is a Mac that ONLY uses SSDs, then it is possible to get SSD replacement cards.

There are companies, such as MacSales.com (OWC) that specialize in upgrading existing Macs including the tools you would need. They have videos to show you want to do, and there are YouTube videos on doing these replacements, along with iFixIt.com guides.

You can fit a larger hard drive to it, you can not increase the size of the existing drive though.