Buckles. Prince International, No. 6, Tulasi Thota, Balepet Cross, Bangalore 560053. Google Maps Location. Multiple sizes. Side-release buckles, tri-glide buckles (of several sizes) & cord-locks (only one size)
For straps. Pavan Trading Company, No. 21, 3rd Cross, J.M. Lane, Balepet Cross, Bangalore 5600053 Google Maps Location. However the quality of straps isn't great. Better to check online or salvage from old backpacks.
For metal buckles, g-hooks & stuff. Muskan Enterprises. On the first floor. Google Maps Location. Thick heavy g-hooks. 3/4 inch & one inch.
It's hard to find buckles in quantities fewer than 50 or 100 pieces.
Details here: https://photos.app.goo.gl/ZmvCesMscZKZTzhF8