Start with a broker like FE International. In the evaluation process, it will help you to think of your business as an ordinary software company.
(I’m assuming you’re not going to sell to a specialist in your open source code itself, because you’d already know about that person from your personal network or customer list. If you don’t know that, try reaching out informally to some of these individuals or companies.)
Potential buyers will want to know what software is sold, how many customers it has, past and current financials, etc.
They will also be interested in what price you are asking and details about how ownership will be transferred.