Thanks to Frankie, we now know there's a nice big Thrifttown out on San Pablo Dam Rd (that'd be El Sobrante). Lots of goodies and a Goodwill just a stone's throw away. There's also an oar store in case you need to pick up something to row with.
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Getting out for a bit? Thrifftown's at 3645 San Pablo Dam Rd, from the city you just stay on I-80 til El Portal Dr and take a right and then a left onto San Pablo Dam Rd. As I said, there's a Goodwill over there too and the who knows what neat things you could find.