How much will be around? Well there’s plenty around but I would say it is not only unwise but extremely dangerous to go delving around private property in search of pot plants.
If you want to buy, there are several clubs in Arcata, as well as it being pretty buy-able on the street. Try the plaza in Arcata for an array of illegal substances.

Marijuana is very, very common, but still technically illegal. I’d ask around the campus of Humboldt State, you’re sure to run into someone willing to sell.