There are some websites that have free MP3 ringtones, such as www.tuneusin.com
and you can do a search here on the forum as well for ringtones. I am not sure why you 8700g makes that beeping sound. I've NEVER experienced that with my 8700g.
Did you purchase this phone brand new? Has the phone been dropped? Did you install an app that may have caused the beep?
If it's like that, then take it back to the T-Mobile store and return it for another 8700g.