Dear Sir, If Dharbam the holy grass is not available, can Sandhyavandanam be performed by wearing Pavithram made of gold?

Madam, there is no substitute for Dharbam. If it is not available, nothing wrong in performing without the same. Yet, the recital values & vibrations of the Mantras are not disturbed because of non availability of Dharbam. Yes, one can perform the daily routine rituals without Pavithram, in case of non availability.