How to find a mature marketing/bizdev parter to join and expand my existing software services company?

LinkedIn is the world's best recruiting source. The advanced search fields allow you so much criteria to build a hit list of potential candidates.

Making direct outreach will likely result in a 20-30% response rate.

I'd look for someone who is less entrepreneurially inclined by targeting people inside similar but much larger companies to the business line you are in.

Best of luck!

Two places to look:
1 - advertise it as a job on Linked In. Yes, I did say a "job". There are real people looking for work who could become partners. I'd suggest you start by hiring and offer the partnership as a performance bonus. That's pretty motivating for a sales/biz dev person
2 - ask your clients if they can introduce you to someone. These folks know, like and trust you and your brand and a private approach from you will carry real weight. They'll be flattered to be asked and you may be surprised who comes out of the woodwork.

Good luck

I would try reaching out in various channels (like Linkedin mentioned above, Twitter or even scouring some meetup groups that might attract the person that might fit the expertise you are looking for) and presenting the value proposition.

Equally, If they have the right alignment ( background experience that speaks to the software business you have-- like experience with SaaS or PaaS or cloud based software companies as a mere example) you could ask them to do some "trial" work to see if it is a good fit (on say a 30-60 period). Most Marketing and Business Development folks are looking for interesting opportunities to solve problems and companies that have good growth potential (this doesn't mean growth to exit like you mentioned, but areas where they can see the business expand in logical ways that fit the business).

Good luck! Happy to help any further if needed.

I would love to talk about this opportunity, seems right up my alley for what I am looking for!

The first thing you will need to do as you begin your market expansion effort is to determine the demographics and the geographic location of the new target market. The number and strength of competitors in the target area. Your market research is an especially important piece of the market expansion process. The more you know about your potential customer base and your competition, the easier it will be for you to identify your niche in this new market. Your data may even help you to decide whether you should focus on this potential market.
You can read more here: https://edwardlowe.org/how-to-expand-your-business-through-new-market-development-3/
Besides if you do have any questions give me a call: https://clarity.fm/joy-brotonath