Always used to use the lugworms (I think that's what they're called) that we dug up along Dunnet Beach.
When you see those little piles of sand that look like worms all along the shore, you will also be able to see indented funnels near each one (like little tiny craters). Find the spot half-way between the two and dig straight down - hey presto! Worms that fish like!
Alternatively, I remember there being and abundance of fields in Caithness... Grab a spade
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