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Only 100 yen or 500 yen coins work, so I looked for some change but couldn't find any.
Literal
100-yen coin [or-か] 500-yen coin [only-しか] cannot-use [because-ので], I [topic-は] small-change [object-を] searched [but-が] is-not-found.
A vending-machine/convenience frustration. しか~ない is the exclusive 'only X (and nothing else)' construction — the しか particle attached to a noun requires a negative verb to complete the meaning. It's stronger and more exclusive than だけ: だけ says 'just X,' しか~ない says 'nothing but X, and that's a problem.' 見当たる ('to be found, be locatable') is a state-ish verb that pairs naturally with 探す ('search') — 見当たらない = 'cannot be spotted,' 'is nowhere to be found.' Note the tense seam: the searching is past (探した), but the not-finding is present (見当たらない) — a vivid real-time narration.