Mentoring Can Be Cross Functional
Mentors don't always need to be experts in what you are focused on. If you use an unusual programming language, that still leaves about 99 non-language topics in software development that you could be coached in. Some of my recent mentors have been involved in business development, hardware, or systems engineering, even though I am primarily involved in software.
Many Kinds of People Can Be Your Mentor
A good thing to look for is someone who is now where you would like to be in five years. A mentor like this is not always available, so other alternative include a sage who has way more experience, but might not be in a formal leadership role anymore, a reverse mentor who is less experienced (or heaven-forbid, younger) but has perspectives on tools and techniques that can be of benefit.
Is Your Mentor Also Your Sponsor?
In addition to mentors, sometimes it is critical to have a sponsor. If you are trying to move up in an organization, if you have a mentor who gives great advice but does not influence your chain of command in your favor, you may be disadvantages relative to someone who has a mentor who sponsors them as the lead, project owner, or manager of a project that earns them a more influential role or a promotion.
Cross Company Mentors and Networking
If you are already the tech director, you may need to look outside your company for your mentor. There is strength in numbers, so networking with many people in a shallow way can be surprisingly valuable. I have generally thought that to be valuable, your engagement with other professionals needed to have the depth that comes from working together or extensive, frequent, or lengthy association.
How Big is Your Pond?
You mention that you have been R & D head and tech director, but you don't feel like you are that experienced. Confession is good for the soul, if not the reputation. I have a similar confession.
At one point, I considered my career to have been divided into two parts. In the first, I worked on a lot of teams where I was either the only software developer or the developer with the most college. The second started when I hired on with a Fortune 100 company and worked with a distinctly higher grade of developer. The contrast was essentially like being a big fish in a small pond to being a small fish in a big pond.
Trading Up
Making the change from small independent companies meant that I had much less status in the organization. But there still was some status. Developers were still valued both with higher pay and a fairly high degree of control over the work. I could also influence fellow team members (although not from the top). There were a lot of role models. Having plenty of competition forced me to toughen up technically and in understanding how what I did provided value to the organization and my teams.
I have previously ignored my gut instinct about roles and mentors and paid a price. Intuition is sometimes a feeling that comes out of a synthesis of experience. My vote would be that if it feels wrong, it probably is wrong and deserves attentive investigation. It sounds like you are concerned about your experience and lack of a mentor, explore your options.
Would you perhaps find growth, satisfaction, and other benefits by migrating to a bigger, more aggressive company? Bigger is not always better, but sometimes has the advantage of diverse people that results in diverse working relationships, and diverse project experiences. Many of the luminaries of Silicon Valley have had or have served as mentors to people who as a result can chart their way to success more quickly and efficiently. One of my classmates from grad school followed the call to join Linked In and is experiencing that culture, playing a much higher level game.
Mentor Benefits
While a mentor can be like a professor, I think that unless you have one who looks at your work product frequently, or serves like a master in the master/apprentice relationship, skill training and professional education are secondary. I think the greatest things mentors share include advice about growth, role modeling of good decisions and balance between workplace values, and steady, objective and sound judgment in times of crisis.
A piece of advice my grandfather gave me many times was that people are only teachable when they are humble. Be careful about statements like "I am way beyond books", and try to learn from everyone. To build a relationship with a mentor, be sure that you are respectful and receptive and that you show appreciation. It will also be important to follow their advice because unless they are extremely patient, they generally will invest their time where it will be turned to action.
