Hi Raghunathan,
It was nice to read your post after a long time.

In Mumbai, and in India, there are many PID controllers available, begining from a price tag of Rs. 1000/- (Approx. US$20) to Rs.8,500/-(approx. US$170).

The one that you want, will cost something around Rs.3500/-(US$70) in Mumbai.
These are mostly made by chinese and Taiwanese companies and sell here under different brand names, such as Vertex, Honeywell, Toho, Fuji, TAIE, etc.

But the ones I prefer are those of TAIE make. I have used hundreds of them throughout India (you know the ground realities here), and not a single complaint from them.

They have Isolated RS485 (Modbus protocol), and have various range of products for different applications. Here is their web site:
http://www.fa-taie.com.tw/en/index.asp

You will find their representatives in Chennai, and if you want one(in Chennai), I will send you his contact number by email, if you ask for it.