1. For daily purchases in Europe, I use cash withdrawals from European ATMs, using my US ATM card, supported by my low-fee credit union.
2. For larger deposits, I use Everbank. They have various demand and CD accounts for a few foreign currencies. US dollar-equivalent principal is insured by our own FDIC.
3. For large transactions, such as hedging my overall savings against future dollar devaluation against the euro (and to the extent that I chose to use the euro instead of the yen as a stable value currency), I use the CME futures contracts. There are lots of brokers, but I use Interactivebrokers. I know it’s a futures contract, but you can use it to effectively convert dollars to euro in a pretty efficient and convenient and reversible way.
