Buying with a credit card has been the quickest and easiest way, but you can only buy bitcoin, then sell, rebuy xrp. The only other way is to do a wire with USD and then buy xrp directly. At the moment it is a hassle, but one day when brokerages like TDAmeritrade start offering, i dont plan to be here...

by the way, i dont think Bitstamp offers credit card deposits anymore.