Cooling System and Water Pump - HELP!

I posted this on the X5 forum, but the issues ate the same for '01 5 with a 3.0 engine:

I have a water leak between the 'female' port on the bottom of the Expansion Tank and the "male" port that it slips over. I see that none of the O Rings are sold separately, this one comes in the tank I believe.....short of a drive around to the local auto parts stores afther I have it all apart........any suggestions? This O ring is about 1-1/2" in diameter.

Also: any DYI out there on the H2O pump replacenent? Other parts taht should be done at the same time?

Thanks in adavance.
Mike