Question

Where (if you can) can you catch Trapinch in Pokemon Soul Silver?

Answer 1

It can be found in the desert of the safari zone by placing 25 peak objects for 30 days.

Comments

You could also catch a Vibrava (1st stage evolution) with 25 Forest Objects after 20 days in the same terrain. Less time, but then you'd need to grab an egg.
Oh yeah, and only after you beat the Elite Four and unlock the ability to add Objects to the Safari Zone sectors.

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what are peak objects?

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Small rock, big rock, mossy rock. Source: http://www.serebii.net/heartgoldsoulsilver/safarizone.shtml and my experience.