The problem you've got here is that you're jsut the wrong side of the part 23/part 25 crossover.
Part 25 aeroplanes are generally airliners - big, heavy, and very expensive: there's little incentive for a manufacturer to go to the massive part 25 certification costs for something only a little bigger than the part 23 limit.
STOL aeroplanes are pretty much entirely part 23 aircraft - which are limited to 19 passenger seats (+ 1 or 2 pilots). This basically means that the big STOL aeroplanes seat a maximum of 20 total.
But if you can limit to 20 total PoB (almost certainly 18 pax in any sensible configuration) then there are several aeroplanes available to you. The dHC-6 Twin Otter would be an obvious choice, but you could also look at the BN2A Mk.III Trislander. On the sort of short hops you're looking at, total payload shouldn't be a particular problem.
As Transaal said, there are military aeroplanes that will certainly do what you're describing - but not with civil certification.
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