It used to be that the Zedge app was a pretty reliable way to obtain notification sounds and ringtones. Unfortunately it became overrun with advertisements at some point, and I now consider it too annoying and cumbersome to be at all useful.
Though the Zedge website still seems to be decent when used from a desktop browser.
There are lots of alternatives, but most of them are junk. Unfortunately whenever you search for "ringtone", you get a long line of people who think they can make money off of you.
Don't get me wrong, I firmly believe that artists should get paid for their work (and I know that many people use that as a simple excuse to justify not paying anyone for the media they consume, but I really honestly mean it). But many of the options that present themselves are intermediaries who want to make money through non-transparent or scammy methods.
On the PC, I like to use Audacity, it's open source and cross-platform.
On a smartphone, some apps I sometimes use include — AudioLab audio editor, Lexis audio editor, (TODO)