Where is a good place to get free ringtones?
The future Mrs. Plant who I dated 1982-1986 wants The Second Time Around. 
I use phonezoo.com; if they don’t have what you’re looking for, you can make your own ringtone, too.
Thanks, Lottie!
If you use iTunes, you can turn any song in your collection into a ringtone for free. There’s no “make this a ringtone” button, but Google brought up simple instructions quickly.
Basically, you set the start and end times to get a 0:30 clip, and then convert the file to MP3. Find the file in Finder (Mac) or Explorer (Windows), change its extension to .m4r, and drag it back into iTunes. It will appear as a ringtone and sync to the phone automatically.
Sorry, Gary, I need to correct your post. You need to convert an MP3 into an AAC file. You do this by right-clicking on your 30 second sample and choosing “Create AAC version.” Then, drag the file to the desktop and rename the extension from .m4a to m4r.
If it is a song you have purchased from iTunes, then it is already in AAC form. In that case, you can trim it to a 30-second sample and change the extension.
Whoops! You’re quite right, Cochrane. My mistake.
What type of phone does your wife have?
Gary and Cochrane already covered the iPhone - you pretty much need iTunes, or some other tool that can convert to AAC.
If she has an Android phone there are several apps that will let you make a ringtone right on the phone.
If she doesn’t have a smartphone, or you’d rather to make ringtones on your desktop, I use a free app called Audacity to chop up MP3s into bite-sized files. It’s capable of fade in/out, pitch/beat shifting, and some other nifty features that can make for pretty cool ringtones.