yes, dry clay can be collected at collection points, but I am not sure where. There are 2 possible locations: top floor of Kelan Cave, west of the east beach in the south part of North island(NOTE: it is still in North island), and is there one somewhere else? Possibly KaMa cave? I can not remember.
I have found a few dry clay before by collecting, if you are struggling to compound them, try making alts and vaccum at collection points, it is more efficient and you never get common stones, unless you need the common stone ;)