MSR Filter/Pump O-ring replacement

Does anyone know where to buy the large replacement o-ring for an MSR Miniworks?  It is the red rock colored ring here.   I don’t really want to spend 30 bucks for an entire msr service kit. I only use this filter once or twice a year when I am in the desert.  My concern is that every time I remove the filter cartridge from the housing, the o-ring wants to simply fall off and hit the ground.  I am concerned about loosing it in the field, as murphy’s law applies to most of my trips.  Thanks