Any ideas for finding a fee-for-service financial/tax advisor?

[A HERETIC I AM]

3. The only thing those types of people have to sell is expertise and information

And since you typed what is basically a contradictory statement;

"I only need one-time advice with ongoing occasional check-ins."

You're asking to have your cake and eat it too!

It's unreasonable to assume a Financial Advisor will charge you one single up-front advisory fee and then be available for "ongoing occasional check-ins" with no further consideration. Even an attorney on retainer is going to charge you additional fees going forward. The reason you are hearing that Advisors want a fee based on a percentage of the assets under management is because that's the standard way of billing investment guidance on that type of account.

The only solution I could offer is this;
When I was a broker and I had a client who purchased a portfolio of Mutual Funds that had an upfront sales fee, then yes, I was always available to answer any questions going forward. There would never be another cent charged to the client in that case (outside of ongoing management fees which are disclosed at the outset), unless they made trades. I got "Trails" or a small annual stipend from the Mutual Fund Company, but on a 6 figure portfolio it was minimal with the funds I typically promoted. I generally stuck with funds that had low management fees.

If any subsequent trades were within the Fund Family, there were typically no costs to the client.

I should add the following;

The best way to find an advisor you can trust is to ask the richest, most well off people you know what firm they use and then interview those people.