Lalitha, seconding what Sindhu and Prasanna said: Medela sells breast shells here, I see them in Landmark (of all the places

) Let me know if you are trouble sourcing them, they have their own unit in Eekattuthangal, can pull ou the address for you.
Was your baby fed formula from a bottle, it could be just the question of nipple confusion, then. Assuming it is so, I would suggest you try to latch on your baby after you express some milk and the letdown or the faster flow starts. She may have gotten used to a faster flow and prefers drinking from "sangu".
The other technique they were planning on using me - but did not have to - is using a syringe to pull out the nipples. I did start out with flat nipples, but with the initial few days of intense nursing it evened out.
I think you are doing good, the learning is somewhat challenging with flat nipples. I have a feeling, the more you put her on your breast (the way you are doing currently, after feeding and after bringing on a letdown) and have her suckle either nutritively or just to comfort her, I think your nipple will pop. Hang in there, things will become easier for you and your LO.
Hugs.