Why is Ethereum total network hashrate still increasing if there are no GPUS to buy? The total Ethereum network hashrate is close to doubling relative to its December level. Mining difficulty is also increasing proportionally of course as the total reward per block stays the same. 

However, it is impossible to buy new GPUs in large quantity anywhere. So, where does the computational power comes from? I can't believe that there was enough capacity used to mine other coins to double the ethereum global hashrate, but I may be wrong. So, where do all the new mining rigs get their power from?
People who already owned GPU before Ethereum became popular are joining mining now.
People with piles of money (companies mostly) buy GPUs directly from manufacturer or big seller (that's why GPU's don't reach retail markets mostly)