The oil pressure switch is readily available from most of the common purveyors of BMW parts, just shop around. I chose to purchase the NAPA equivalent. Here is a link to some alternative sources for BMW consumable spares. http://advrider.com/forums/showthread.php?t=207573
While I've never had any issues with mine, the oil pressure switch is a critical component in that you really need to know if there is a problem with the oil pump immediately. Be guided accordingly.