If you google 'sari shop' you will find loads of online stores that sell saris, suits and anything else you might like.
Find out about the ceremony, it might be in a hotel or in a temple or gurdwara. You need to know the rules of how to sit.
Don't wear black or white.
For my friend's wedding I wore a long dress to the Sangeet, for the henna party I wore jeans as did most people other than the bride who was having her feet and hands done. She was keeping her hands wet with lemon and water - the darker the henna goes indicates whether your mother in law loves you. My skin goes dark brown by the time it is dry.
For the Gurdwara I'd made a pair of baggy trousers with a long jacket, very similar to this pattern www.pinterest.co.uk/pin/295408056781036246/
But I bought the material from an Indian material shop and made a headscarf as well. You will probably need to cover your head at some stage so worth thinking about.
For the evening I had a sari I'd bought in Mauritius a few years before.
If you buy a sari from a shop ask them to show you how to tie it and make sure they sell you the underskirt as well as the - no idea of the propper name- but a piece of material you stich inside the sari - it makes it hang better.
I made my own blouse to go with the sari.
If you go to an Indian material shop you can ask for a recommendation for an Indian tailor.
Have fun both getting the clothes and at the wedding.
One word of warning, beware 'the aunties', these are an almost mafia like collection of older ladies who may or may not be actual relatives.
There purpose in life is to fit as much food into someone as they can, and all the relatives know this so they home in on people who are not of Indian /Pakistani descent and feed you. You you have had breakfast, then lunch and someone made tea and insisted you had samosa with it and now it is 4.30 pm so you must be fed again.
Also, steal the grooms shoes. You will not be suspected.