you sometimes see him at the place where you see the gymleaders.
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He should be in his house.
He is in one of those houses in the resort area
Buck lives in the Survival Area in Pokemon Platinum. Buck is part of a quest involving Stark Mountain and Heatran.
Heatran can be found in Mt Stark, you need to have gone through it with Buck first, after he return the Magma Stone you can then travel through into where Heatran is, where you faced Team Galatic before.