Google pointed me in the direction of an old blackberryforums.com post by xVidia, who had posted a list of Bold ringtones in mp3 format. Surprisingly, the link was still valid.
If you don't know already, just create a folder called "ringtones" or "notifications" depending on how you want to use them, and drop them in.
Honestly though, right after I had it done I realized I had already gotten used to the stock Droid sounds and abandoned the idea completely. Just thought I'd share.