Bittrex is good, as mentioned, but you need another exchange to send your euro for buying some bitcoins first. Then you can send these bitcoins for example to Bittrex and/or Bitfinex.
For buying bitcoins I wouldn't use bitcoin.de as they don't protect your data well (they cooperate with German authorities even if there is no judical decision, yet). Therefore you may use Bitstamp instead.
RE: Investors Pull Billions from Stocks As New Bitcoin, Crypto Options Appear