Hey, it worked for N*Sync & the Backstreet boys!
How many of the American Idol winners do you think play an instrument?
I can only offer these bits of advice:
Find people who play instruments in your city or nearby cities (local newspapers, churches, etc may be good places to check). Ask them to play for you. Get to know them. Tell them your situation, and ask if they'll help in exchange for something. Maybe offer them a little money or something for their services.
The harder, more time-consuming route would be to learn to play an instrument. Best way to go here would be piano or guitar.
Of course, you could always buy Karaoke tapes of your favorite artists/groups and make "demo" tapes of that, but I don't know what the legal implications are.
There's also taking the acapella route.
Maybe check your area for talent contests. Enter them and see how you do. Many of them will either let you sing a capella, or with a backup band or Karaoke music.