You can usually find Spanish fly in adult themed shops or possibly on market stalls. It can be difficult to find depending on your location though, so the internet is a safer bet.
I found a website called Mediherbals who deliver the product all over India. You can order by phone, or there is an order form to send your enquiry.Please be cautious, and make sure that you are happy that the product you are buying is from a legitimate company before parting with your money. It all looks okay, but it is hard to be sure. I'm just a little concerned that orders can't be placed directly from the website and it does not state which payment methods they accept.
Please be very careful with Spanish Fly itself, as many variants are actually fake and hardly do anything...
Yet this can sometimes be preferable to the real stuff. Overdoses are horrendous and can do so much harm that even a slight overdose will result in an erection that will need medical help to get it to subside and any more than that can lead to death...
My advice would be to read articles about Spanish fly before purchasing. This post i very informative: http://io9.com/5983275/why-spanish-fly-only-works-on-men--and-is-deadly