Its not possible. You can't buy telephone ringtones to install on your iPhone.
You need to use the ringtone installation procedure from iTunes.
Your iPhone will not install anything that you would normally have downloaded on your old regular cellphone -- no games, not ringtones, no screensavers or wallpapers.
Everything goes through your iTunes.
Suggestion = read user manual. If you are new to smartphones, then you really need to read -- it won't do the things you want it to do based on old experiences with regular cellphones. You really do need to read the manual if you want to understand the iPhone.