You can buy them directly from the market. Alternatively, you may try to farm YFII following this guide.
Please refer to this guide.
WETH is wrapped ether and the price is always 1:1 to ETH. You can swap ETH for WETH on Uniswap or Balancer.
DAI is a stable coin anchored to US dollar. You can buy them from exchanges or DEXes such as Uniswap and Balancer.
You can buy LINK from exchanges or DEXes such as Uniswap and Balancer.