As much as I like to think there is a first-world solution to this, the reality is that the best supplier/consultant leads always come from word of mouth IME. Engineering societies, linkedin, etc are good starting points but regardless you need to get out, meet folks and ask.

Tech support issue is much related to the overall quality level of vendor, manufacturers who care much about their brand would be sensitive to client dissatisfaction with tech support.

I had lot of such problems, but you need to describe specific problem to allow any advice.

It's not that different than trying to find in-house talent; we like think that we all employ the best and brightest, but the reality is that there's invariably one person that's the go-to person for any given subject, and that one person is in demand way more than any other person in group.

TTFN (ta ta for now)
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There are websites that specialise in connecting expertise with companies in need. Freelancers and consultants are registered with them and either available to view or connect with directly or via a request submitted to the site. I'm cautious about posting the links here but a quick google search should turn up something relevant.

It seems to me that often enough a very experienced engineer or even technical sales person working for the types of vendors you mentioned sees the problems you are experiencing with several companies, sees an opportunity and starts working independently to provide that extra level of support. My experience of people like this has mostly been in industrial and manufacturing areas for example support with problems in a paint line. Usually I hear about these people from the vendors so could be worth asking

I have to say though that word of mouth and power of the network are still the best options.

Declan Scullion CEng
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Check with people who rep for the manufacturers in your area. Sometimes they have a PE on retainer or can recommend someone they do a lot of business with.

I used to count sand. Now I don't count at all.

I look for people who have published something or presented on technology I need help in.

Professional Societies like A+SAE, ASME, SME are a good place to look.

Talk with a consultant or design firm you have used for another discipline.

I recently had good luck with the local university consulting on a project.

For components sometimes a big distributor has engineering support that is less biased. They still recommend one of their lines but with multiple lines you get more options.