Dear Madhav,
The pictures of 24 names are given in the blog below:
From your question, I presume that you are looking for the 'udaka-pramANa' while doing Achamana. As per the various smritis, the pramANa is that of the 'size of single black gram'. Well, most of the smArtAs use this 'purANa Achamana'. Yes ! there are variations in certain parts of Tamilnadu. Some chant only 12 names instead of 24. And, some take 3 sips of udaka by chanting 'namaH" and not 'svAha'. However, we chant 24 names and initially the udaka-pAna is done with 'svAhA' only.
There are certain rules associated while doing Achamana and it is better to learn these from qualified purohita or vedic teacher. One should face either East or North while doing Achamana and the posture should be something that of 'kukkuTAsana'.
It has been observed that most of the brahmins are unaware of the Achamana-vidhi and during evening sandhya, they turn West and do the Achamana. This is a wrong procedure and our Acharya used to scold and used to say that one should do 'sa-chEla snAna' if accidentally performed that way. One should not do the sound while sipping the udaka; for brAhmaNAs, the udaka should pass through hrudaya to nAbhi sthAna and only then 2nd sip should be done.
So, these aspects should be taken care off while doing sandhyAvandana which should be meticulously followed. As soon as upanayana is performed, one should be trained in sandhyAvandana, Agni kArya and followed by brahma-yajnA. Unfortunately, the plight of today's brahmins are deplorable.
regs,
sriram