You should know that you need to have a data-capable phone, and there will either a per-month charge or a per-KB charge to download the file over your cellphone's network.
If you're sure your phone will play MP3 files, then download the ringtone to your PC, then use this site to upload the file to your phone:
http://rumkin.com/tools/sprint/index.php
This will upload a copy of the file to the Rumkin servers, and send you a text message with a link that you can use to download the file to your phone.
If your not sure whether your phone will play MP3 files, then use this site:
http://mobile17.com/
You do have to register and provide your e-mail address, but registration is free, and I've never received any spam from them.
You'll need to specify what brand and model of phone you have, and what carrier you use. Once that's done, click the "Mega/Super/Real Tones (with Lyrics)" link on your main page.
Choose the "I'll download this file with my phone's web browser. (typical)" option and enter your mobile phone number. Then click Continue. This will upload a copy of the file to the Mobile17 servers, and send you a text message with a link that you can use to download the file to your phone.
Select your mp3 file by clicking the "Browse" button at the top of the next screen. Label your file, and choose where you want it to start - for this file, you'll leave that as the default "0 minutes, 00 seconds".
This file is a hair over 39 seconds long. so where it says "4. How long would you like the tone to be?", enter 40 seconds. Finally click continue.
This site takes longer, but the reason it does so is that it converts your mp3 file to one that's compatible with your phone before it sends it to you. That's a boon for someone like me who's got a Sanyo SCP-8200 that doesn't play MP3 files.
Hope that helps,
-RBB