Although you can purchase installation media, the beauty of Linux (and with it Debian) is that it's (usually) free in more than one sense of the word. Liked freedom? You will love free beer too. Debian is free in both ways (See notes below).
Obtaining and preparing an installation image is relatively easy, and you have plenty of choices here too.
That said, purchasing installation media helps the project. Consider it a way of donation, if you like. Or if you don't want o buy stuff, you can just donate to help the project, if you can. Donations help to keep Debian free.
OK well, if you are feeling that generous, why not just start with helping to keep this website (or at least the Debian handbook on it) ad-free? The author of this site also accepts donations, you know, which will hopefully pay the bills for hosting this site, so no advertisers need to be involved. Just click the donate button in the sidebar, if you wish to help.)
Noticed the pun? The last notice was labelled "Tip"...