I am assuming that you are asking about source of funds for a student visa application. If that is the case, the source of funds is actually secondary to the funds being there themselves. CIC doesn't have any business in how you make your money, they just want to know that YOU have the money and you are not just showing it for a short period of time. Payslips are needed to show that you have a steady income. If you do not have payslips, you can always write in a piece of paper the monthly incomes and outgoing of your business.employment.
You can also produce an employment letter which states your salary and if possible state in the letter that you are paid in cash.
Hope this helps.