You can order from them according to the prices they quote or you can choose your own sharksfin and they'll quote you the price. The second shelf of the stall is the fresh sharksfin. They lay it out on these flat rattan pieces (if you peer hard enough, you might see the fins which are the lighter brown frilly things on the rattan "plates").
And here it is! This was the THB500 bowl. It came served piping hot in a claypot bowl with a side of raw beansprouts and a very healthy dose of greens - parsley and spring onions. On the table is the omnipresent picked chillies, fish sauce, etc the Thais so love!
I've tried both - those at the stalls and those in the restaurant and we've reached the conclusion that they're both pretty similar. The main difference, of course, lies in the prices (restaurant ones can sometimes be up to THB3,000 to THB6,000) and the size of the fins (restaurant ones are the real pao chi - thick pieces of fin).
Apologies to the activists out there...but this is one real lip-smacking treat!