Without Text nor Internet no, the ringtone cannot be purchased from a website (internet is a definite requirement as the tone would have to be downloaded. The link is sent as a Browser message but I'm pretty sure you'd need text). You could always try making your own, but I don't know what phone you have to give better detail on what program would be needed to send it to your phone via USB cable (or bluetooth).
In that case all you should need is a data cable and either a memory card or the program to access the phone's files. If you have a memory card, you can transfer the MP3s to your memory card and if it's required to be on the phone to apply as a ringtone, bring the MP3 file up on your phone and press Options, them Move, and Phone. I'm not seeing anything as far as length specifications, but you may want to keep it under 30 seconds (that's about as long as a phone will ring before going to voicemail).