Locality is most of the value
A specimen without a locality is decoration. With one, it is a data point that can be compared, catalogued and trusted.
Dealers who cannot tell you where something came from either do not know or would rather not say. Both are reasons to spend the money elsewhere.
Stop paying for size
Beginners buy large and regret it. Thumbnail and miniature specimens of good quality cost a fraction of cabinet pieces, store in a drawer, and teach more per pound spent.
A shelf of twenty well-chosen miniatures is a better collection than three large mediocre pieces.
Assume treatment until told otherwise
Heating, irradiation, dyeing, oiling and stabilising are all routine and mostly legal. Failing to disclose them is the problem, not the treatment itself.
Ask directly and in writing. A dealer who is evasive about treatment has answered the question.
Where to buy
Shows are the best starting point, because you can handle material, compare prices across a room, and ask questions of someone who has to stand behind the answer for the rest of the weekend.
Online buying is fine once you know what you are looking at, and photographs consistently flatter. Ask for a scale in the image, ask what the back looks like, and treat any listing without a locality as decoration priced as a specimen.