For bulk sourcing and export quality products, Ludhiana (Punjab) and Tirupur (Tamil Nadu) are key town in India where shirts among other garments and textiles can be bought. In Tirupur there are a lot of supplier, which sell based on weight!!
Having said the above, you should make use of trade site like Indiamart, alibaba, tradekey among others to find suppliers for you needs or attend trade exhibitions..
If your requirement isn’t for mass quantities, you may find vendors, closer to your location, supplying at the same cost as the factories or those who can supply on consignment basis.