Since deblocking filter module conducts filtering on the reconstructed pixels, you can refer the source code (TComLoopFilter.cpp) to see how to access the reconstructed pixels.
TComPicYuv* pcPicYuvRec = pcCU->getPic()->getPicYuvRec();
Pel* piSrc = pcPicYuvRec->getLumaAddr( pcCU->getAddr(), uiAbsZorderIdx );
pcCU->getAddr() means that index of CTU
uiAbsZorderIdx means that zindex (might be 4x4 block index usually) in a CTU