It is not a must to buy the 100 onions from an exchange. A 100 onion balance in your airdrop wallet is just one of the requirements to join the airdrop. Othere rules are here: https://deeponion.org/airdrop.html
You can get the 100 Onions with minimal effort. You can get them by joining the official Deep Onion forum and get DeepPoints that are converted to Onion coins every Sunday. You can also get them through the bounty program (language translation, video creation, tutorial creation and article writing). Buying from an exchange however is the fastest to get those 100 Onions.