If you filed a tax return with your full income last year, that will often suffice. Or you can offer to show bank statements with regular deposits.
Or you can call potential apartments, let them know you run your own independent business (feel free to make up something -- concierge service, petsitting, babysitting, yard work), and your income is primarily in cash, and just ask what they'd recommend for proof of income. That's probably the easiest way.
If you really can't come up with anything, offer to pay several months' rent in advance. That can be very convincing. Worked for a friend of mine.
If you haven't filed taxes, I highly recommend doing that this coming year. Tax evasion is a serious charge! I like TaxAct for doing mine -- I'm a tax dummy and love that it walks you through each step. http://www.taxact.com And Gina of P411 recently started recommending these folks: http://www.companiontax.com If you owe back taxes, it would probably be worth your while to consult a professional.