Hi Eric,
I used to see them at an LFS in the Clementi area, not C328, but I think the shop closed down. Maybe you should try and find landscaping stores, or those plant farms. They should have slate pieces available. I think C328 stocks them as well, but the sizes are not fixed from what I recall.
Alternatively, if you want the caves just for pleco breeding, might as well buy straight from Aquabid from this guy called ZCliff.
http://www.aquabid.com/cgi-bin/aucti...ing&1267138801
Some of the Pleco guys here own some of his works.
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