Activate your pop-up stopper. Type your phones's manufacturer and the words "free ringtones" (with the quotes) into Google.
Other than that, it really depends on your phone model. My current Nokia 3390 comes with a composer built into the phone, so I can use [url]http://www.indiskret.de/klingelton/ton_index.html[/url] and type the tones into the phone manually, which looks like this:
(2)# 28 1 2 2# 5 (6)9# 68 5 6 6# 2* (4)99 08 (4)8** 38 2 3 4 6 (1)9* 78** 6 7 1* 3 (5)99 0
Speed 125 bpm, press and hold the numbers in parenthesis.
That's the theme of Das Boot, btw.
For other models there's a PC suite to do this on your desktop PC, but then you would need either an infrared connection (on both the phone and the computer, laptops already have that, some phones do to) or a data cable.
German kids and Europeans in general are nuts about their toyphones, so "Klingelton" would be the German keyword to look for. But pay attention: many websites try to install dialers on your computer, nasty programs that replace your default internet connection with one that dials a number somewhere in the Caribbean for $50/minute or more. Those programs don't work on US computers, but they are a pest anyway.