Extremophiles - bacterial that can survive and thrive in extreme conditions of heat or cold. The DNA of these bacterial suggest that they are very ancient. Dust grains - grains of interstellar dust contain ammonia, methane and water and more complex molecules like methanol and ethanol and so also may play a role in the creation of monomers. About 3000 tons of dust fall to Earth from space every year carrying organic material. These meteors contain amino acids.
Some monomers come directly from space.
Production of monomers - there are a variety of processes on Earth and off that could have provided the monomers necessary for life. Synthesis of polymers - requires input of energy and not too much water since the formation of peptide bonds ejects a water molecule. Too much water will reverse the process and dissolve peptide bonds. Proteinoids - chains of amino acids that form when amino acids are heated, but which do not necessarily correspond to proteins involved in life processes. Clay - layers of silicates and water may provide catalytic site for synthesis of polymers by holding amino acids while they bond.