Amplifying Millivolts with a Single Power Supply Op Amp in a Large Gain
I can't speak to component availability in your area (Sri Lanka?) but there are many op-amps that will do this. Look for Auto-Zero or Zero-drift op-amps. You can easily get relatively high accuracy with a gain of 100 or so for 100mV to 1V out using a simple non-inverting amplifier configuration.
For example, AD8551 which has a maximum Vos of 5uV and a maximum TCVos of 40nV/K. Voltage noise 0-10Hz is 1uVp-p so that should not limit the ADS1115 too much.