How do go about finding a mentor to help me take my software company from 6 figures to 7 figures in annual sales?

This is nit-picky, but I think reframing your question will help you get the result you need.
Having a mentor and figuring out how you will pay them feels like a contradiction since I've never had a mentor that ever asked to be paid for any advice they gave me.
Finding a mentor is a very organic thing in my experience, and definitely worthwhile, but I think the shortest path to getting to a 7 figure revenue rate is to get a consultant that can help you improve your skills and better focus your resources.
You could do this by leveraging them in a group setting, like masterminds, you could retain them monthly, or just about any other format they're open to.
I've got a client right now that is in a very similar spot as you, doing $350k and just needed some guidance to get better focused on what will get them to $1MM. Our arrangement is with a weekly call and he's asked for more help, so we're moving my deliverable work under a profit-sharing agreement.
I'd use Clarity to talk to a lot of different consultant's and see what you find. Just don't jump into an long-term agreements until you know they're a fit and are more interested in your success than their fee.
Practice with me for free if you'd like and I can help you get your questions dialed in better for future calls: https://clarity.fm/joshualong/VIP

A mentor is usually not someone you pay....they are usually someone who has a specific skill set or experience you are missing and they can help guide you in your next steps.

I building my business, my mentor had already been through everything that would happen as I got started. She was instrumental in keeping me on track and helping me understand the obstacles. Her support and past experience was invaluable.

Before you start looking for mentor create a list of the skills you need to develop. Knowing that you want to grow sales exponentially, what specifically do you feel is missing for you to make that leap and how will your mentor help you accomplish this?

A few ways to find a mentor include through your local Chamber of Commerce; use your LinkedIn contacts and groups to seek out who has the qualifications you require; a tech accelerator and business networking group may help here too. Sometimes you will find that your mentor may not come from your field of business....it's the experience that you are looking for to help you grow. They will be able to guide you around obstacles, anticipate what could come at you and help you formulate your future steps. This other perspective is helpful in providing insight in new ways to your business.

Please let me know if you would like to discuss mentoring more. I am happy to have a call to bounce other ideas around.

There are a ton of different ways to go about finding a good mentor. Ten Thousand Coffees, for lack of a better analogy, is like a dating site for local mentorship and networking. It streamlines the natural cycle of mentorship by letting you list your profile and browse a database of professionals who are open to networking or mentoring the next generation. Networking is easier in-person for a lot of people. Be sure to check out these networking tips before you attend.
You can read more here: https://www.shopify.in/blog/how-to-find-a-mentor
Besides if you do have any questions give me a call: https://clarity.fm/joy-brotonath