Yes that's a common issue right now, only a few crypto-currencies can be bought with FIAT.
To know which coin you can use to buy onions (or any other coin) you can go to coinmarketcap in the Markets tab: https://coinmarketcap.com/currencies/deeponion/#markets
Here you will see all the trading pairs and exchanges.
As you will see onion could be traded against BTC / LTC / DOGE only on Cryptopia.
But a new exchange Kucoin will add DeepOnion soon so stay tuned!

Anyway investing in DeepOnion seems a good idea congrats! :)