You'd probably be best served to just go through an investment bank. FX market is relationship driven unlike the stock market. You don't have any of the trading

You'd probably be best served to just go through an investment bank. FX market is relationship driven unlike the stock market. You don't have any of the trading relationships with the huge players in the market, and probably never will. The cost in time and money to participate in these markets vastly outweighs the paltry sum the traders are making on your transaction. Solicit quotes from multiple counterparties, pay attention to the institutional rates, and make sure you aren't getting screwed too badly.